We have probably done this before but meh.

Crysis 2
Soul Edge
Panzer Dragoon Saga
Virtual On
Freedom Fighters
Deus Ex
Turrican
Dragon Age: Origins
Crysis
Warhammer 40,000 Dawn of War
R-Type
Command & Conquer
Hitman: Contracts
Universe at War
GTA: Vice City

WOW reaction : Outcast. Nothing else.

Now, that being said, there are countless of soundtracks that I really love : Deus Ex series, Planescape, HoMM 2-3 and a lot more.

Already added Deus Ex ;)

I'm nominating Hitman: Contracts and Command and Conquer Generals, as well as Anarchy Online and Painkiller. The Silent Hill games have an incredible soundtrack as well.

I don't know if it counts but GTA soundtracks are WOW in my opinion.
In Vice City, there is even a song called "Wow" by Kate Bush!
http://www.youtube.com/watch?v=PPN0BDtmKZQ

While that is a great track I had to give it to Flock of Seagulls,

As crap as the rest of the game was, the soundtrack for Alone in the Dark (2008) was amazing.

Quest for Glory, especially Quest for Glory I. Mark Seibert did some great stuff with that.

While I don't care much for the game itself, the MUSIC from "Mass Effect" was absolutely phenomenal, especially the main theme.

And for one of my favorite game intros in a long time, "Borderlands." Quite possibly the only actual song in the entire game, but it fits so well and sets the mood for the game perfectly.

Age of Conan's "Ere the World Crumbles", because Basil Poledouris' piece just didn't work out for me. I agree, Ain't No Rest For The Wicked was very catchy.

Sonic 2 was also the precedence to awesome OSTs for future Sonic games (especially Sonic 3 and S&K)

I like Tropico's soundtrack...makes me feel like I am on a tropical beach sipping on a margarita or pinacolada

The BEST SOUNDTRACK EVER is by far the soundtrack of Morrowind.

Lands of Lore: Guardians of Destiny was the first soundtrack I bought. I was pretty disappointed to find that the CD was actually a selection of the music, and that my favorite piece was not included.

I know Im not supposed to talk about the game here ;-), but Halo has an epic soundtrack.

Morrowind's music was okay, but I much preferred the Oblivion soundtrack. The tracks just took on a much more epic feel in the transference.

As for mentioning Halo, well, I don't have a problem with it. A lot of my friends would agree with you.

Oh! Another game that has always stuck with me as far as the OST: Legend Of The Dragoon for the PS1. The opening is just beautiful, and the music through out the game is subtle, but still wonderfully mood-setting.

NFS:Underground :)

Okay, Half-Life's sondtrack was very fitting, quite sublime. Also HoMM 3, stuck in my head for almost twelve years.

I've always been partial to the music from Hired Guns.

And if you want to go even earlier, Target Renegade.