Any co-op game, like Splinter Cell, where the other players don't take it too serious. Me and my friend play Bob and Steve! I do get serious when I go multiplayer in Tales of the Abyss (I have too, I play as Natalia).

And fighting games too. Soul Calibur 3 (Sophitia or Celsius) or Budakai Tenkaichi 3 (Trunks or Videl) or Ultimate Ninja 2 (Hinata or Temari) I play mostly.