Great question. Not a big video game guy, so my choices would be stuff from my youth. Ocarina of Time and Donkey Kong Country 1 & 2 are soundtracks I can still listen to because the music is so good. I also like Super Mario 64. I'm guessing people already mentioned these, though. To be a bit more original, let me put Quest for Glory V's name into the running.
There's some beautiful music in this game. Check out a couple of pieces.
My favorite piece in the game. Gorgeous.
Another nice one. It plays when you're just walking around the town, which has a sort of Roman/Aegean setting. The music is perfectly fitting.
This one's just plain fun. I actually used to play it on piano once in a while, ha.