Depends on the game. In TF2,I`m actually halfway decent with most classes. However,lately I`ve been performing best as an ambusher Pyro.
In CoD,a pair of P90's for CQB,a thermal scoped and prefferably supressed sniper rifle for siping/countersniping (shoot,move! I NEVER camp!),or an assault rifle with a reflex sight for general combat.
In Crysis...? Cloak->Ambush position->prone,Maximum Armour->Spray enemy troops. Repeat if overwhelming is plausible.
Also,if it's possible,I will ALWAYS aim. Unless in Crysis I'm using a laser sight at close range. The red dot does the aiming for you.