My friend pulled off a good one on MW2.
This guy was about to get a care package (we didn't know until after the match) and he killed him, then threw this one onto chat:
C-C-C-COMBO BREAKER!!!!
Me, my mate and 3 people on the chat broke up into laughter.
Mine is, as I usually play Age of Empires 2, and occasionally win, I use one of thed in-built chat things to signify entering, obliterating, and defeating them:
RAIDING PARTY!!!!
Ohh, I'll send you back to the Age of Empires!
Oh sure, blame it on your ISP...