Let's see...
-rock band 2 (didn't like 3's main setlist much)
-bit.trip 2: future legend of rhythm alien (though I had to look up the full name
).
-kickbeat: steam edition (fighting+rhythm=win
)
-140 (though you can argue it's a rhythm game...it's mostly a platformer)
-audiosurf (though it may be I like it because it was the first rhythm game that let me use my own music)
-piano master (a free-to-play android game without a catch...and is just fun attempting to play music)
But if I have to pick one favorite, I'll go with the following:
-duet (both great on PC and mobile...and an absolutely AWESOME soundtrack)
I'm sure there are many great ones out there (I really want to try rocksmith, but don't have a budget for a real guitar, and I'm very curious about crypt of the necrodancer...or osu, for that matter), but I just haven't tried as much as I want to.