Most of my favorites appear in the following:
Deus Ex
Freespace
Freespace 2
Unreal
I also like some others, such a few of the "action" samples from Medal of Honor: Allied Assault, the Ground Control briefings, track 7 on the Descent 2 Vertigo CD, and a couple others.

To tell the truth, the only reason I like the concept of music in the first place is because of these great game soundtracks; I'm not into popular music at all.

And the debriefing (Crayven side) of Ground Control simply rocks...
I don't think I have heard the debriefing one, but both the Crayven and the Order briefing musics are fantastic in my opinion. In fact, I put a section of the Order one into my FS2 campaign as a briefing music.

Actually, I think the best music so far is Act On Instinct from the very first Command & Conquer.
I remember that one; it was pretty nice too. My favorite in that game was "In the Line of Fire."
