i have lots of favourites, but the main one is the SAW gunner in Full Spectrum Warrior 2. If there is like ten enemies pinning you down you just select him and go "Suppressing fire" - he stands up and unleashes hell with his weapon and sends enemies running for cover.
also the level 3 incineration in bioshock, just because he clicks his fingers to shoot it.
Gravity hammer/Spartan laser (the second because it can instakill those pesky brute chieftans)
Flashbangs in ANY game in co-op because it pisses them off so much, EXCEPT halo 3's flare, i can't seem to use that without getting blinded myself, and its useless in campaign, i don't have xbox live, so i don't tend to use it that much.
Also, and don't shoot me down in flames for saying this, what is a BFG?