After some weird phone call from a friend, mine says "I kill you. Peace."
Funny enough, I made this back when the 360 first came out, though I didn't have one myself. Even still, Xbox.com allows you to make a 360 profile anyway, and that's what I put in there a couple years ago.
Now, it won't let you put "kill" in your motto, so I'll probably never change it just because of the novelty.