Of course, I think Ridge Racer Type 4 has to be up there. I still find it kind of surreal that a racing game was given such a funky soundtrack. It's soooo good. I can't think of another soundtrack that makes me feel the way that one does.
Racing Lagoon, MOON, Katamari (1 and 2), Gran Turismo 4, Earthbound, Space Funeral, and Halo 2 or 3 are my runner ups.
And to throw a modern game into the mix, FF7 Rebirth; the sheer amount of unique music alone was staggering, but the fact that most of them are bangers.. it made playing the game so incredible. It must have been such an undertaking to put that one together.