I like both orchestral-type stuff and techno in a soundtrack, so:
Force Commander--Dawn of War. This is epic.
http://www.youtube.com/watch?v=ECMpaEQ0MFU
Vortal Combat--HL2: Episode 2. Accompanied by, in my opinion, the most badass fight in the series.
http://www.youtube.com/watch?v=swM2fWu9cnA
Access the Animus--Assassin's Creed. I don't really have a favorite track in AC, but I like all the music in there, the interesting mix of traditional and techno, and the eerie and mysterious nature of the whole thing.
http://www.youtube.com/watch?v=G0DlLEo8T7k
The Halo series also possesses a great soundtrack, despite my other issues with it. And while we're at it, I've never heard anything by Jeremy Soule or Jesper Kyd that wasn't awesome.