Gears of War: First time I killed General RAAM, I used the Troika machine gun. When I beat him I was treated to a close-up shot of him getting his legs blasted off and landing on them in a pool of his blood, face down. At first, I thought it was scripted, but when I couldn't get it to happen again, it was all the sweeter. A gory end to a gory game.
COD4: I REALLY hate snipers, so I take great pride in single-handedly cleaning out a whole nest of them in Bloc. I killed three with my Uzi 9mm, and when the thrid one downed me, I thought it was over. However, I was able to waste two more on Last Stand because they were so absorbed in sniping my teammates that they didn't even notice my rampage. Very satisfying.
GTA4: I was fighting Johnny Law with the RPG when I died and went into the slow-motion death sequence. That wasn't the end of my spree, however, for the RPG went off, destroying a police crusier which had pulled up beside me and sending me flying. Hilarious.