Mine for the moment is super smash bros melee. Its a great game for many people like for a party. It is extremely well made and I never seem to get tired of it. Im still trying to get all the trophies. I have 236 =)
I refuse to decide, in fact, it's almost impossible to decide considering that there are so many different types of games. My favorite FIGHTING game is either Soul Calibur 2 or SSBM.
I have favorites in other genres too. Tales of Symphonia is probably my favorite RPG followed by Baten Kaitos: Origins.