Good old WAAAGH!!!, especially when someone else gets it.
Or the other one, CRY SOME MOOOAAAR!!!
When I used to play Guild Wars, we had either "GO GO POWER RANGERS!!!" or "We are Koreans, give up already and save us time". Or just "WE'Z ALL GONNA DIE, WE MUST RESIGN"! Then 7 people write /resign (when everyone in the team of 8 does that, the team looses automatically) and enemies are really, really confused.