Hmm.. this is about thousandth post about gamemusic..... BUT, I must answer one again.
Here are some of the most enjoyable soundtracks in my opinion:
Hitman 2: silent assassin
Freedom Fighters
Diablo 2 and Lord of destruction
VtM: Bloodlines
VtM: Redemption
Deus Ex
Fallout 1/2
Quake
Quake 2