There's too many to list, so I'll just narrow the big ones down:
Sonic CD JP (Sega CD): It doesn't sound like something you would normally associate with a Sonic game, or really any other game at the time. It sounds more like actual music done by professionals, only significantly shorter. Although there are only 7 core level tracks, they each have four versions that sound vastly different from one another. It's excels in both musical craftsmanship and remixing. No other game had music like this back in the days (early 1990s), and they still wouldn't have that many years after the Japanese version of Sonic CD. It was an OST that was way ahead of its time, and in my personal opinion, is still miles ahead of many OSTs out today.
Tetrisphere (Nintendo 64): This the OST that got me into electronic music. Neil D. Voss is the most ingenious composer in video gaming that still has only a cult following. The Tetrisphere OST is a melting pot of electronic music with everything from techno, house, trance, break beat, big beat, ambient, and even chiptune a la Game Boy midi style. Even to this day, it's my favorite OST 12 and a half years later. It's hard to believe that it all came from one brilliant mind.
Embodiment of Scarlet Devil (PC/Windows): It's not as diverse or engineered as the other OSTs I have listed, and is in fact rather simplistic, but it shows what ZUN, creator of the Touhou series and the only member of Team Shanghai Alice, truly excels at: catchy, pretty, addictive melodies. Ranging from calming to silly to intense, EoSD is earworm at its zenith encompassing a large range of atmospheres. For most people in the states who found out about the series, they have this soundtrack to thank. It's not only fantastic, but also a gateway for many that have dared to walk across the bullet hell-ridden border.
Katamari Damacy: An unusual game with an equally unusual soundtrack. It traverses a variety of genres ranging from electronic to jazz to samba to rock ballads to pop. It's a soundtrack that took just as many chances as the game itself, exposing itself to the public not knowing how it would be judged, or just how well it would be received. Silly and quirky while being complex, mind-bending, and immune to growing stale. Although the later games had great music as well, it will never top the game that started it all.
