Beatmania IIDX: Empress + Premium Best (because AFAIK it has more songs than any other IIDX release) with a fancy-pants DJ Dao controller. Or else one of the arcade versions of IIDX.
The game has a near-infinitely steep mastery curve so even if I'm not playing anything else, I think I could play with a sense of challenge for a loooong time. Plus I really like the music.