For me, I'd say the biggest one would be Septerra Core. That game has a lot of repetitive battles, but I just love the battle music (thanks Marty O'Donell). I'm not sure I'd rank it anywhere near my favourites if it wasn't for its music.
Also, I doubt I'd have been able to beat Super Hexagon without its music getting me pumped to dodge random lines and shapes. In fact, I probably would have given up pretty quickly otherwise.
Oh, and the early THPS games - particularly #2.