On maps that I don't lag like balls in (silly orange maps, and a few others) I'm pretty decent with the spy. I walk past an enemy, do a quick snap to the side and backstab him.
On other occasions, I play as an offensive engineer putting sentries in the worst but decently effective places possible at the enemy's point. For defensive, I usually camp, unless there's another engineer. I build up with him, and run out to shotgun some people while he defends the farm.
I play soldier a lot, too. I used to suck at aiming, till I changed the crosshair to the tiny cross. Much more ownage, and I end up using shotgun more. The tiny cross also helps with Scout, Sniper (no-scoping ftw), Spy's revolver and Engineer.