Castlevania: Symphony of the Night for PSone has some of the best videogame music ever IMO. It helps that it's one of my favorite games of all time. But it has a very varied soundtrack, it's great. You can find a youtube playlist here
but I'll link to some of my favorite individual tracks.
Lastly the ending credits theme which I think is brilliant. Foolishly replaced in the Xbox Live Arcade version. >:|
The other videogame music that comes to mind first is from Mirror's Edge. The theme "Still Alive" by Lisa Miskovsky is awesome and so is the ambient soundtrack used throughout the game.