My XBL is "TheUnknownNoob" which I always thought was clever... some better ones I've seen though:
"Tech Support"
"Kenny you bastard!" (In C.O.D 4, such that if you shot him, it would read "YOU KILLED KENNY YOU BASTARD!")
"TEAMMATE bob" (In C.O.D 4, such that if you killed him, it would display as if you had teamkilled.)
"MY MASSIVE ERECTION" ... yeah... that one was fun.
"Kanye West" I'm real happy for you, and imma let you finish...
"a puppy with cancer" ... You killed a puppy with cancer... you sick ****er...