Its not really funny but very creativie in my opinion: Combat Wombat. it rhymes Another one i have seen is: bigbonaforu
I love the people that think my GT is a drug reference... all you gotta do is google image my GT and look at the image to see what it actually resembles. Spoiler
It was modded; and way overdone, but the one that oficially made me lol the most was "Press (X) For Sex"
Can't remember anything flat-out funny at the moment, but I was playing MW2 a while back, and saw someone on my team get a kill with a Claymore. His GT was BOOM Hi
The funniest would either be "Poop Sprinkler" or "Yo Mommas Nuts". Im not sure that those are the real gamertags, but it was close to that.