best game music i've ever heard if from Black Mesa source.
http://www.blackmesasource.com/
its not even OUT yet and its awesome!
Final Fantasy 7/8, Metal Gear Solid 1/2/3.
Unreal Tournament had an outstanding sound track.
Quake. Music by NIN
I'm afraid I can't let the Sonic games on the Mega Drive go without a mention. The Donkey Kong Country series also had some great compositions
(I do love lots of other game tracks though, but some have been mentioned).
FF8 and FFX, the music from both brings back some fond memories for me. Infact it makes me wish I could go back and in hind sight I would appriciate them much more.