>>723683105
>ME1
Pretty much interchangeable, also got the multi classes which mingle them together.
Generally though Biotics > Soldier > Tech powers. Biotics CCs everything anyway, soldier lets you kill things faster and even faster with warp.

I'd say Nemesis > Bastion/Shock Trooper/Commando > Operative > Medic.
>ME2
Sentinel > Infiltrator/Soldier > Vanguard > Engineer/Adept.

ME2 tech armor is the most busted ability in the game. Stronger shields, stronger powers, instantly restore shields after they break, breaking creates a pulse that staggers and knocks down enemies, resets your squads cooldowns, does fucking everything. Class also has access to warp and overload.
>ME3
Back to being pretty interchangeable cause everything's busted.
Engineer/Infiltrator/Vanguard > Soldier > Adept/Sentinel.

I would single out engineer because AI hacking takes over Cerberus Turrets and Atlas Mechs, which are dangerous in the early game. Grissom Academy becomes a cakewalk, with your other drones and squadmates you can split enemy aggro like 5 different ways while their turret mows them down.