My favorite game music, regardless of how much I liked the game would be:
Grim Fandango - Peter McConnell's best work ever. A combination of mexican salsa, moody violins, swing, bebop and jazz, all with a strong sense of thematic coherence. It's very difficult to get fifteen thousand elements and mix them together in a manner that makes them even remotely palatable, but somehow McConnell did it.
Ghost Recon - No, I didn't actually get to play it, but it's the best reproduction of bombastic Hans Zimmer like scores I've ever seen. And in a game about a super-secret and elite task force that just as much belongs in a Jerry Bruckheimer movie, you can't ask for anything else.
The Myth Series - The first two installments didn't really have much in the way of music, actually. Music was only heard outside of missions and mostly in the briefing dialogs that came in the form of journal entries written by one soldier in the war. With somber piano and several subdued instrumental elements, the music accurately portrayed the feeling of helplessness and hopelessness in an possibly unwinnable war.