well I haven't been able to play online yet, but I remember some funny ones from Call of Duty 1 online on PC some years ago (yes, way way off topic, but not ENTIRELY)
like...
"You were killed by A Poor 4 Year Old With One Arm"
"You were killed by a gun"
"You were killed by a three headed flying carnivorous Platypus!" (mine, at one point)
"You were killed by YOU!!!" or "You were killed by Yourself"
"You were killed by Sam Fisher!" (from Splinter Cell)
there's some more but I forgot
Also they're as funny or almost even funnier when it says "You killed Sam Fisher!" (which never happens because Fisher is a BA), or "You killed YOU!"