ME1:
Infiltrator - Snag Hacking as a BP and you have all the core tech powers, armor, immunity and range. True the pistol is the only other weapon but with marginal work, it can compare well to an AR.
Soldier - All but impossible to kill with thickest armor, most choice in guns (though I tended to stick to AR/Sniper) and HP.
Vanguard/Adept - About the same to me, though a Bastion Adapt is totally unmatched in a one-on-few fight, the vanguard can hold out against a larger number of foes.
Engineer/Sentinel - Still have not played these classes in ME1 *shrugs*
ME2
Infiltrator - While not quite the fully self-sufficient class as last time, it can own most fights in short order, as long as there are enough clips.
Solider - Would be top, but I just like the infiltrator more.
Adept - Irritating to need to pull most of the defenses off at higher difficulty, it's still satisfying to make boom in the faces of the targets.
Engineer - A key for (nearly) any lock. Get Reave and there is not defense the engineer can't strip. Killing the target, on the other hand...
Vanguard/Sentinel - Not completed ME2 with these two classes
ME3
Engineer - OMG. My engineer is rolling face on insanity, cryo burst makes all the difference in the world when you see a Brute die in under 10 seconds. Drone and Turret are nice, but (Overload/)Cryo/Incinerate will wipe out things in seconds with the right power choices to balance off of. (Note that fighting /alone/ is a drastically different ballgame. *points to the first phase of the last fight in Omega DLC*)
Infiltrator - Bias showing here most likely, but as my primary character playthrough, I still like and had only slight difficulty with. I will admit to relying a lot more on squadmates then from previous games though.
Soldier - Still going strong, but not too much so.
Adapt/Vanguard/Sentinel - Not played yet, though the Adept is waiting once the Engineer run is done and plans are in place for a ME1-ME3 vanguard run after that.