These days? Probably Super Smash Bros. Brawl and Dance Dance Revolution/In The Groove.
Brawl, just for the sake of trying to beat my friends. I just fight way too wrecklessly though.
Since I'd rather play In The Groove at my arcade, I've been attempting to move up to 12 footers as of lately. It's tricky since it requires so much stamina, but for easier songs, I could act like an MLG fag since I'm one of those people who cares way more about accuracy than showing off to beat a hard song. Anything I'm comfortable with I usually get 97% or higher.
And Project Justice? I guess I could add that also. Bring it, I'll use Shoma, Roberto and Hayato on you.
[edit] I just remembered this one.
Tetris Attack/Panel de Pon. I LOVE this damn game.
Jam it back in, in the dark.