SO many to choose from...there's so many bits and pieces from loads of different games that I love, like The Sands of Time, Diablo, Morrowind, Baldur's Gate, but the soundtrack for Knights of the Old Republic 2 is consistently brilliant. KOTOR 1 is great as well, but having a live orchestra for the sequel makes it incredible. The battle music is really lively and varied enough for the different environments, but its the quieter moments that are just so good. The creepy music for Darth Sion, Korriban and the ghost-ship 'Harbinger' are so atmospheric, and 'The Jedi' is perhaps the best of the all, so wistful and melancholy. Its pitch-perfect for the sadness and introspection of the whole story.
In case you haven't guessed, I'm obsessed with soundtracks.