What is, in your opinion, the best video game/computer game music you have ever heard?
Earthworm Jim

Hmm... Not bad for a retro/comical theme.
Bullshit. What game had the most epic score? Either KOTOR, Oblivion, Morrowind, or Mass Effect. All had great scores.
Are you only considering the epic orchestral themes? Or also more rock based themes or things like that?
sonic adventure battle 2, or sonic adventure 2 battle... whatever. had a great soundtrack. i just loved the main theme.
Crysis : Warhead, amazing gameplay, music, graphics and all around beauty. If you have a decent system enough to run this game you'll understand.
Dynasty Warriors series, for sure.
All of the Dynasty Warriors/Shin Sangokumusou soundtracks perfectly harmonize traditional Chinese instrumentals with Western rock. It's an interesting combination.
Also, the four Onimusha games include quality traditional Japanese orchestral scorings from four talented composers, with the fourth game also introducing rock elements.
I don't know how, and I don't know why, but this is totally Sheep's fault.
/thread.
Edit: Jeremy Soule is also pretty awesome. He did the soundtracks for KOTOR, Guild Wars, Morrowind, Oblivion, and a few other games I can't recall offhand.
Dark Forces 2: Jedi Knight. John Williams knows his stuff. =p