1) We stand no chance. In ME2 it is established that the firing power on an Everest class Dreadnaught's main gun is equivalent to that of the bombs dropped on Hiroshima and Nagasaki - maybe more powerful, I forget.
It takes continuous fire from four of these things to take down the barriers of one Reaper Capital Ship. From this fight my estimates are total annihilation of Earth, 60% due to collateral damage, with 0 Reaper ship losses (Do remember they could also likely shoot down most any Nuke we fire at them).
2) Reaper Victory. Don't get me wrong, some Reapers would die, but really Man stands no chance. Well, if Master Chief was on our side maybe.
Actually, do we get the Halos? If so, I'll call it a draw. Reapers are somewhat Organic, and thus should be killed by the Halos right? So, activate the Halos and everyone dies. Otherwise, Humanity would lose. Reapers use Rush tactics and blow up all ships and stations with their murder lasers, and Humanity is F***ed.
3) Reapers win. These guys fair better than the humans, but they still get their butts kicked. Somewhat interesting battle, and Reapers would suffer a fair few casualties, but the Reapers would win in the end. One or two shots from a Reaper, enemy ships dead. You've seen how fast Harbinger attacks.
4) Close fight. I'm willing to say the Protheans actually stand a chance. Highly advanced technology, secret research projects, a galaxy spanning empire - the Reapers would have a fight on their hands. In the end I'm going to say the Reapers would win. Hit and run on the undefended, taking up Protheans and indoctrinating them or turning them into Collectors, then sending them against their fellow Protheans, or as sleeper agents into Prothean settlements. It would take a few years, but the Protheans would crumble.
5) Deathstar. Destroys a planet 1 shot. Its weakness? Cooldown.
Star Destroyers and such? Bleh. Dead, easy. I'd give them maybe 6 shots from a Reaper to kill them? And seeing as they tend to hang in relatively small fleets from all I've seen, just Mass Reapers on them and slowly destroy the fleets one by one. Hell, the biggest collection of Ships I've seen in Starwars was probably at the Battle for Coruscant - and the Reapers would just tear through that fleet. I'll hand victory to the Reapers here. Sure, they Death Star is powerful, but it can be outmanoeuvred and then destroyed.
6) Victory to Organics. Maybe. If all the Reapers at once vs this... Victory to Reapers likely. Seeing as the Reapers enjoy dividing and conquering though, Organic victory. Deploy massive fleets wherever Reapers are sighted - MASSIVE fleets - and retake the weak systems. Seeing as there are a lot of fleets, you could probably defend the whole galaxy, or give pause to Reapers attacking before a defence fleet arrived. Proper tactics employed, Organics + Geth victory. Proper tactics not employed, Reaper victory.
Now, for my entrant into this fight: The Vasari from Sins of a Solar Empire. Shields? Check. High Hull, and nanites that repair the hull? Check. Missiles that phase through space and can BYPASS SHIELDS ENTIRELY? Check. These guys also had an empire spanning the majority of the systems and planets around the galactic core, rather than the sparse colonisation across the galaxy seen in ME. High tech. Secret research projects. And the Kostura canon. This thing disables all systems on anything it hits. Hit a Reaper fleet, they're stuck drifting whilst your fleet annihilates them. Oh, and a Phase gate network, allowing their entire fleets to be mobilised to any planet at a moments notice. Not like the Mass Relay network where you have to go from relay to relay to get somewhere. With this network, anywhere leads to anywhere else.