I sdon't think anyone has gone for the good ol', [HEADING=1]"LEEEEEEEEROY JENKINS!" [/HEADING]
But other than that, I will go for "Cthulhu fhtagn!" or...
If I'm feeling a little bit nerdy and uncreative, I go for "MAGGOTS!"
If I'm feeling lik a '60s man and want to tune in with my avatar, I'll shout "I am not a number! I am a free man!" (cookie for whoever guesses the cultural reference) and then when I'm about to execute-kill the last guy, it'll be "Be seeing you."
But my general purpose war cry is "NYAAAAAAAAAAAAH!" accompanied by a very unhappy looking cfieldplatt