Yes it is hard to give a precise definition of power. I tend to want to use a nebulous assortment of factors from time to clear most levels, strength and weakness vs the various factions, synergy with the weapons, while assuming a relatively experienced player. Also I will ignore the Mattock completely, since I don't have it and really it is its own balance issue. One confounding variable is the power curve. The different classes gain power at different rates, and some level off much faster. Maybe I will address that later. So here is my list for now...
Vanguard
I personally don't think you can beat a veteran played Vanguard in this game, with possibly the exception of a Soldier using the one broken tier gun. Some conventional wisdom holds that the Vanguard doesn't beat out other classes unless it has Stasis but I don't entirely agree. As far as leveling goes, you are fairly strong with 2 or 3 powers fully evolved and a 1 pt wonder bonus. Damage Protection 2 and 3 are very helpful, of course with Redundant Field and Hard Shields.
Sentinel
Late game Sentinel is probably the hardest class for the enemy to kill. Because of TA reset you can practically get by with Assault Armor, maxed passive, and a bonus. Pretty good at finishing combos, doesn't have a way to set them up. Does have the best power recharge bonus in passive. The main reason I rank it below Vanguard is because it only has the "caster" starting weapons loadout.
Soldier
Soldier is somewhat unique in that it only has one active power worth investing in, one of the worst active powers, and then the best three ammo powers. The passive has zero recharge bonus, but since you don't need recharge it isn't a big deal unless you are doing atypical builds, or running with a bonus power. You get the most starting weapons, but biased a bit towards anti-armor. Assault Rifle training isn't a huge advantage initially if you don't have the Mattock, except that you simply have one extra ammo pool.
Without the Mattock, you end up with an actual choice between Revenant, Claymore and Widow. The bad news is that the Claymore and Widow don't quite mesh as well with your trademark power as with Charge or Cloak respectively (or even power-reload Incinerate on Infiltrator). Especially the Claymore, which is most effective reload-canceling away (impossible during ARush). But the Revenant / Viper combo is probably one of the strongest 2 gun combos that is possible in the base game, especially with a couple weapon upgrades, and they both work beautifully with Adrenaline Rush.
You have less mobility than the Vanguard, less real durability than the Vanguard, but on the whole pretty solid overall. The soldier doesn't have amazing synergy with any squadmate, but on the other hand is less dependent on squadmates for defense stripping or combos.
Engineer
This is where it starts to get murky in ranking the classes, or perhaps it started one class up. In any event, I am picking Engineer for the next class because it is the only class that has good CC combined with all defense stripping without any bonus power.
Compared with the above classes, you need a little more talent point investment before it pays off, but you can counter-balance that with squad choice. Also has "caster" starting weapons and relies on CC or a bonus power to deal with lack of or add durability.
Adept
Adept needs a little more power investment than the Engineer. Of all the classes it probably has the slowest ramping power curve, but once it is leveled to near the top it can be one of the more destructive classes and a solid controller. There might be a compelling argument that mid to late game Adept is superior to mid-late game Engineer. Maybe someone else might argue that.
There is only one innate specific defense stripping power, although Singularity works well in some situations. The fact that you need to rely on several powers stresses the cooldown system. There is flexibility in squadmate choice, but it can be argued that it requires good selection to play closest to potential.
Infiltrator
Of course this section is murky, but I am going to double down on this one anyway.
The main strength of the Infiltrator is single target damage. Mid to late game this translates into good elite or miniboss killing. However, you must pay for that during the early game where you are only decent at single target mook damage, which isn't a stellar or unique asset. The Mantis is a hindrance here (its ammo capacity is one of the great balance mysteries to me of ME2... maybe they purposefully wanted to depower the Infiltrator).
The build progression of the character is also a little whonky. You do have access to one of the better ammos, and slightly earlier than Soldier. But it needs to be fully evolved before it has good CC. You do unlock a decent organic CC power after a few ranks of Cloak, but it isn't great CC unless fully evolved. Cloak Rank 1 is probably one of the weakest Rank 1 powers in the game.
I realize you can change position with Cloak, but you can do that with Charge, Adrenaline Rush, Tech Armor... and Drone or Singularity aren't bad for CC until you change position either.
A decent bonus power goes a long way to fixing your woes. Alternatively, you can consider in getting Incinerate before you level your ammos or possibly even Cloak.
