For me it's a 3 way tie.
The first is the soundtrack for Halo 2. I'm not really a big fanboy of the Halo series(hell, I never even owned an Xbox, only borrowed), but the music from that game is pretty damn good and really fits the game well. An example:
The second one is from the first Age of Empires, the cool thing about this soundtrack is that the two composers, Stephen and David Rippy tried as much as possible to use instruments from the time periods that the game covers. An example form this game:
And last, but not least, the music from the Hitman series. All of the games features amazing music, hell, in the second game a real orchestra was used to create the music. The example I've chosen to use though, is from the first game, more specifically the main title. It's just pretty damn cool: