I played through ME1 with every class, but ended up preferring the Infiltrator for higher difficulty replays. Same with ME2, only this time I preferred the Soldier switching between sniper rifle and machine gun. As much as I like the idea of the Vanguard in both games, I just can't ever get into it, and giving up my own combat potential just never works for me so no Adept, Sentinel, or Engineer other than my self-imposed mandate of trying them once. It's not that I can't get into the tech and biotic powers, I just prefer to have them manually activated from my squadmates while I lay down covering fire. This way I can pick and choose what powers I bring on a mission, while always being guaranteed of having solid firepower on my team.
Edit: Plus, it's a fairly universal truth that overwhelming firepower is a viable solution to any problem.
Edit: Plus, it's a fairly universal truth that overwhelming firepower is a viable solution to any problem.