1.) Sentinel
2.) Engineer
3.) Tech Armor
4.) Incinerate
Sentinel is from my experiences one of the most capable classes in Mass Effect 2. Incredible durability, can take down almost any armor and given a shotgun or sniper rifle does that very well too.
I think it is too 'hybrid' however and prefer fully Engineer or fully Adept although I find Adept lacking compared to Mass Effect 1 and thus my second answer is Engineer.
The best ability is definitely Tech Armor, although not a favourite as I don't really enjoy being the 'shield' much. Even on higher difficulties this really saves your life constantly, you can take more damage and you can instantly restore your shields with this if they go down normally.
Most fun is, imo, incinerate (Though I don't even like fire that much

) as you see them scream and burn and constantly take damage. It is also incredibly effective in almost any situation.