Warning: LONG post ahead… Sorry… Interesting topic…
It’s a bit difficult to answer since the action of the main character could very strongly affect the effective outcome of the war: even if the galaxy ends up defeating the Reapers, the actual outcome for the races depends on the single Shepard.
If I had to go with what we know now however, my opinion is as follows:
Council -
Humans (4 to 4.5): Humans have good adaptability, have a relatively quick generational turn over and - we are told in the game – are highly genetically diverse when confronted with the other races. They are also creative and capable of thinking “outside the box” (the inventions of medigel and carrier-class starships are good examples). All this would definitely favor them when the time to rebuild comes forward, they’ll be more ready to adapt, rethink and restart.
On the other hand, they are (apparently) the main focus of this Reaper invasion and since they have been in the galactic community for quite a short time: most of the population and the majority of the industrial, political and military complex remain on the homeworld, Earth.
Since we know already Earth gets invaded at the very beginning of the game and isn’t retaken till the final battle at the endgame that gives the Reapers a long time to cripple the planet, and if Earth gets crippled hard enough it remains to be seen how long it would take for us to recover.
-
Turians (4): Turians are deeply committed to the cause, each and every of them feels first and foremost a public servant, as such great displays of power are only going to harden their determination in crushing their enemy (we saw it with the Krogan Rebellions). Furthermore, they have a military oriented head set and the strongest military fleet of all the Council races, they had time to spread to several worlds (they had already colonized several planets by the time the Asari discovered the Citadel) and their society is extremely meritocratic (that means that as a general rule whoever leads is competent to lead, though there are exceptions of course).
On the other hand, their dextro amino chirality puts them at a disadvantage when it comes to find new colonizing prospects, should they lose some of their worlds, and they have to protect their client races (like the Volus) in addition to themselves.
Also, we know the Turians have evacuated Palaven due to the Reaper invasion. This may not seem like much, but Palaven is the seat of the Hierarchy, the Turian central government; we know the Turians have already experienced civil war back with the “Unification War”: in the day the Hierarchy kept itself neutral while the colonies fought it out, then quickly conducted a sweep and conquered them all when they were weakened enough due to infighting.
However, even after near 2000 years some colonies still hold a grudge against each other and the “Facinus Incident” on Taetrus (from CDN) shows scissionism is still present, if the Reapers weaken the Hierarchy too much the Turians may very well splinter back to warring colonies.
And since Turians fight a war with the objective of totally obliterating the enemy’s military and in Turian mind set everyone supports the military and is a public servant ready help when attacked that could cause them to effectively pummel themselves to extinction.
-
Asari (4 to 4.5): The Asari are natural biotics, their society is tightly knit as shown by the fact they are effectively many different republics and city states allied together but we are never informed of any actual civil war among them. The skill of their commandoes is legendary in the galaxy as is the biotic power, the intelligence and political skill of the matriarchs who lead their society, also all the Asari are at least informally trained in the use of biotics and small arms in the defense militias of their respective towns (who are made of volunteers). They are probably the most widespread of the known races and their long lives makes it so they have perspective in their plans all the other races lack (apart from maybe the Krogan), which will help in rebuilding.
Finally, they can reproduce with anyone(they basically clone themselves in hutero and mix match half the alleles with useful genetic traits “scanned” from the partner DNA through mind melding), meaning that probably as long as one or two Asari survive the race won’t go extinct if there is at least another race still in existence (unless the Asari survivors are Ardat-Yakshis).
On the other hand, their long lives are both a blessing and a curse: it seems to encourage complacency, if Matriarch Aethyta is to be believed, and the Asari apparently don’t become sexually mature till they actually reach around 40-60 (this age is given considering the Ardat-Yakshi trait manifests with maturity and that’s the estimated age Morinth run away from Thessia), and they don’t feel the need to start a family till they are Matrons (should be around 300-500 years old), this would mean recovering from substantial losses would take a LONG time for the Asari.
-
Salarians (4.5 to 5): Salarians are known for their intelligence network and their non linear thinking, both important traits to survive and rebuild, also their approach to war makes it so they always attack first, so they probably won’t find themselves totally unprepared. Their rapid metabolism also colors their culture making so they actually “move, talk and think fast”, all this is essential for response to a threat such as the Reapers and the inevitable need to restart after everything has ended.
Finally, their Dalastrasses lay a dozen eggs every year after they reach maturity. Assuming they have proportionally shorter times to reach sexual maturity that means fast recovery: let’s say a dalatrass is mature at 6 years and dies exactly at 40 years (but 40 is an average so they could live less but also more), 12 eggs every subsequent year means a total of roughly 410 new Salarians from every dalatrass (41 of which are new dalatrasses).
Given this rate of population recovery even if they are a bit pummeled in the war they should be up and about in a much shorter time than the other races. True, their actual military doesn’t particularly shine in any way and their short lifespan sometimes causes them to lack perspective, but unless the Reapers totally annihilate all the Salarian females, or the Union collapses to infighting due to power vacuum, the race will recover.
Not Council -
Volus (2 to 2.5): So long as the Turians last, the Volus would probably recover, since the former would probably shield the latter making it possible for the Volus not to feel the brunt of the Reaper invasion themselves; in this case the number is probably the same as the Turians.
If the Turians fall however, or find themselves incapable of protecting them, the Volus would not last: we are told time and time again how their race is not physically adept at fighting and as a general rule has a non violent society, they may be able to protect themselves in space using warships (that’s assuming, as a client race, the Turians even allow them to have an independent military), but if the Reapers land they are pretty much done for…
The fact they need extremely specific environments to survive (aside from the pressure, they are the only known ammonia breathing species) makes it so they’ll have a hard time recovering from the loss of any of their planets. To survive they’ll need specifically made environmental units and domes, but one has to wonder whether they themselves or the other races would have resources to spare to build and maintain said units and domes.
-
Vorcha (5): The Vorcha ability to adapt to any kind of environment means they could basically go and live everywhere, so as has been said, unless the Reapers plan to scan every single planet in the galaxy, they are unlikely to get all killed, and their society is used to having a lot of its members killed before their time anyway (they constantly fight against each other, since they haven’t caused their own extinction yet, that means they naturally recover well from these losses).
Also, there’s the fact they are effectively a pre-spaceflight species (the ones we see around the galaxy are supposed to have been brought there by “Blood Pack” mercenaries or to have snuck into the cargo of some ship to move somewhere else), most of them living on their overpopulated homeworld in constant infighting.
If it’s true the Reapers ignore pre-spaceflight species (which would explain why we humans survived the previous onslaught) then the Vorcha would probably survive with minimal losses.
-
Yahg (5): Same as the Vorcha, they are pre-spaceflight, roughly on par with 20th century human tech for now, so if the Reapers ignore pre-spaceflight they survive without any losses at all on Parnack. As an interesting side note, I’m half certain if we lose against the Reapers the Yahg will become the next cycle galactic spanning civilization (like the Protheans).
-
Hanar (2.5 to 3): Their ideal worlds are oceanic (80%-90% ocean covered surface) and humid, there’s not that many like that, so a lost world means a lot to them. While they appear to be capable of living on land, that’s mainly because they use mass effect fields extensively (they would be unable to sustain their own weight otherwise), that’s not feasible in a post apocalyptic galaxy with resources now all spent on rebuilding, besides even with mass effect fields they move rather slowly, to be at their best they need to stay in water.
Their worlds of origin are also a blessing though, since the Reapers themselves will have very few places to land on and it’s unclear how they could reach the Hanar underwater (Can the husks swim well enough to battle a Hanar in his natural environment? In fact, can they swim at all? Do they even breathe?), unless they go on a rampage and just bomb them from orbit, but that’s not going to earn them anything, given the Reapers normal modus operandi such a solution seems wasteful.
So in the end they may have a chance, if anything they seem resourceful: any species capable of building starships without having digits has my respect…
-
Drell (1.5 to 2): There’s a few billions of Drell still on Rakhana, but they are pre-spaceflight species so they are stuck there in a constant state of civil war over rapidly dwindling resources, these Drell are said in game to be basically doomed to extinction, Reapers or no Reapers. Those that the Hanar saved are very few in numbers - around 500000 if I’m not mistaken – making them the least numerous race in the galactic community (followed by the Quarians at 17 millions and the Vorcha who are always unaccounted for since they are seen as vermin by basically everyone).
Given they were just saved by an extinction, having to fend off another isn’t exactly a chance: most of them live on Kahje now, so they live or die based on the destiny of that planet and by extension the Hanar. One thing is certain however, they are already dangerously low on people and losing more won’t help their chances.
There’s also the problem of Kepral Syndrome: I doubt the Hanar, for all their good hearths, would have resources to spare to adapt the Drell after the war, so the malady is going to hurt a lot Drell’s recovery (assuming they manage to recover at all).
-
Elcor (3): I’m very unsure about this one. The Elcor only special condition on planets is high gravity, but they are fully capable of living in lower gravity ones too if need be (with perhaps a bit of help to prevent low gravity bone loss), so they shouldn’t have too many problems relocating.
The natural worlds they inhabit could be a good natural defense for them: the husks we saw so far weren’t exactly agile, so unless they are specifically made for high gravity they’ll squash after their first fall; as for the Reapers themselves, we know they can land on normal gravity worlds despite their size, but can they do the same in worlds with 4-6 times that gravity?
What’s to consider however is the effect on Elcor mentality… Their high gravity worlds colored their psychology, making them deliberate and cautious, uncomfortable with sudden changes and, well, nothing is more sudden than giant starships popping up everywhere killing and harvesting all they get their mechanical tentacles on. The enormous shock and psychological backlash could very well cripple Elcor society for several generations, hampering the recovery process.
-
Batarians (3.5 to 4): Batarians have definitely a strong military, they are no longer aligned with the council and no longer bound by the treaty of Farixen, that means they can build as many dreadnoughts as they want, and the fact they are the only thing maintaining a façade of order in the wild Terminus Systems means they must be good sweepers, who strike fast and hard. Their paranoid government, the Hegemony, keeps all Batarian space in an iron grip, making extensive use of spy drones and satellites, so if the Reapers come they’ll probably notice in time.
The fact said government has also direct control over all weapon factories and branches of the military makes it so maintaining order shouldn’t be too much of a problem. Their caste system also ensures everyone knows his place and his duties, and overstepping one’s place is extremely frowned upon, that also helps in maintaining order.
The big problem for Batarians are the Batarians themselves, their government is extremely isolationist, even ignoring the fact fighting the Reapers on their own would be suicidal, if they survive, the heavy controls on all sort of goods entering and exiting their territory is going to hurt economical recovery.
Furthermore, we know little about actual Batarian society since the Hegemony has a Department for Information Control, so we totally ignore what the average Batarian thinks of his government.
Assuming there are dissenters (most slaves are for certain), the weakening of the military following the Reaper wars could cause rebellions to spark throughout Batarian space: having to fight a civil war after the destruction the Reapers caused and simultaneously having to keep an eye on the Terminus could pretty much devastate Batarian society.
-
Geth (5): Little to say here, Geth are immortal, their memories and stored data are transmitted to the nearest network database and archived when a mobile platform dies. This data can be later downloaded back to another mobile platform as needed. Even if the Geth where to be pummeled back into non sentience, their original programming as VI allowed them self optimization, so by simply downloading more data in other platforms to work “better, faster and more efficiently” they’ll eventually reach critical mass and “wake up” again…
It’s my belief that anything short of the total annihilation of the Geth neural network and the Geth memory databases would eventually cause the Geth to come back… Maybe in 1 year, maybe in 10, maybe in 1000, but they’ll be back.
It’s also interesting to note the Reapers don’t seem to want the destruction of the Geth anyway, they want them rewritten as obedient servants so they can be used as grunts and drones, most likely replacing the Keepers as the citadel custodians. The Geth as machines are more controllable than the Keepers that as organics evolved in unpredicted ways (so says Vigil), it also explains why Sovvie wasted time creating a virus to rewrite the Geth, but then didn’t wait for it to be used, it wasn’t to have more troops, but to have obedient servants after the victory (otherwise it would have waited before attacking to have all the Geth with him, what’s a few more years in a cycle of 50000). Sovvie may also have “tested” said virus on its own troops, to see if it worked, would explain the strange way the Heretics act (according to Legion).
Regardless, the Geth will probably survive, whether we win or not.
-
Quarians (1 to 3): Yeah, the range is big, mainly because the Quarians’ survival hinges totally on whether or not they manage to claim a world of their own, doesn’t matter if it’s Rannoch or not, but they need a world (and a mainly dextro amino one at that). It is speculated in Ascension the Quarians can live at most another hundred of years or so as they do, since their ships are simply too old, they are breaking down faster than they can fix them.
It’s also important to note that the Quarians are down to 17 millions of people more or less, all their ships are jam packed and overstaffed, the Liveships like the Rayya in particular are the pulsating hearth of Quarian society and they are massive (possibly dreadnought size).
The whole fleet protects them, but it’s not like they can hide and a lucky shot could basically destroy a sizable chunk of their society (given their situation, having all that DNA disappearing from the gene pool is not a good thing), not to mention the fact all the food for the fleet is produced there.
Like has been said, if the Quarians are to commit themselves to war without risking extinction they need to shelter the non combatants somewhere, if they can’t do that the probability of them recovering after the war (or even just surviving it) is pretty slim. If they can do that (and the Reapers don’t attack the planet they are on) their fleet becomes an advantage: all their ships, while old, are heavily protected and armed, and the Quarians had 300 years to refine the art of ship combat and their flight coordination during fleet operations.
-
Krogan (2 to 3.5): Another big range, the matter here is the Genophage. Given Krogan organ redundancy and natural regeneration it’s not unfeasible for their race to be capable of holding their ground against Reaper troops. In fact they’ll probably be more than capable of holding back all but the strongest types of husks. However, unlike back in the days of the Rachni Wars the Krogan no longer have the numbers for long attrition wars, and even if they had the Genophage prevents them from replenishing their numbers fast enough.
Krogan society has already being “dying” for a millennium now and Krogan have, as a general rule, gotten rather fatalistic: rather than trying to survive, they embrace they are doomed and fight with even more reckless abandon; this together with Krogan’s generally inflexible style of combat gives them a severe disadvantage: every Krogan that dies is one less left to repopulate.
There’s also the fact their race is relegated in a DMZ, selling Krogan weapons or armed starships is illegal, as such the Krogan (with the sole exception of the Blood Pack) have no fleet of their own, so unless someone “helps them up” they’ll only be actually useful when the Reapers invade Tuchanka, but by that point their help won’t matter and they’ll be probably doomed (if the reapers land on Tuchanka it means the Turian Fleet guarding the DMZ has been obliterated so the Reapers have total orbital control).
In any case, whether or not the Krogan have a viable future after the war depends mainly on how the Genophage thing is handled.
-
Rachni (3 to 4): Ignoring the fact they could be extinct already (and had it not been for this fact the best case scenario would have been 5), the Rachni have many advantages: aside from living in worlds that are considered not habitable by every other race (apart from the Krogan), making relocating a non issue, they can basically replenish their numbers as long as one single queen survives: the queens carry the genetic code of their father as an emergency and can thus reproduce without the need of a mate if need be, also they can lay eggs in a few hours and can have a colony in a few days.
So basically they can easily outbreed everyone (again except maybe the Krogan from the old days), are naturally difficult to kill and their males, the Brood Warriors, are the only creatures – other than the Asari – we know to have natural biotics.
However they have a crippling weakness, their hive mind: the queen shapes the minds of her children, by “singing” to them she literally teaches them HOW to think. So that gives the Reapers an easy way with indoctrination, for other races everyone needs to be personally brainwashed, for the Rachni gaining control of the minds of the queens is enough: as she “sings” to her children an indoctrinated queen will shape them with the distorted way of thinking the Reapers desire, without forcing them to indoctrinate everyone themselves.
This glaring problem alone nearly outweighs all the natural advantages the Rachni have as a species; in fact, either a solution is found to this or its absolutely vital for the queens to stay out of the way at all times…
For if they get indoctrinated they’ll have to be wiped out again, and that’s assuming certain factions are ok with having them back to begin with.
So this is my opinion, sorry for the long post… ;P