Damn, all the ones I would have mentioned off the top of my head already have been.
Oh well, I'll just tell the person who said something good about Oblivion that they're wrong.
Hey did anyone mention MDK or Giants: Citizen Kabuto? Those had good soundtracks.
Also, if we're taking console games too. Persona 3 has a pretty good soundtrack once you get over your initial repulsion at the Japanglish lyrics and the general silliness of it all.
The Xenosaga series has a good sound track too, especially the Episode 2 theme song. Best thing about Episode 2 actually considering how mediocre that installment was compared to the others. Don't know what language it's in, doesn't sound like Japanese.
http://www.youtube.com/watch?v=JwdmKVb2dgw
Also, Ico and Shadow of the Colossus have really good soundtracks that fit what's happening in the game really well. Or at least SotC does, I can't remember Ico's soundtrack that well.
eg:
http://www.youtube.com/watch?v=y4UvBI9vADc
http://www.youtube.com/watch?v=R3PRXFLJYcw
Rogue Galaxy
http://www.youtube.com/watch?v=bCggbQQLhkE
For it's time Gundam Side Story 0079: Rise From The Ashes had a really good soundtrack.
http://www.youtube.com/watch?v=fsUXLzjw7dE
While I'm rattling off PS2 games, the Lu Bu theme song from Dynasty Warriors is pretty bad ass too. Though I wouldn't call it a "best soundtrack".
That's all I can think of now.
EDIT: Just thought of another one. Phantasy Online Intro Song. http://www.youtube.com/watch?v=_56oGUrvgO0
EDIT Again, Erg! Somehow I forgot to mention Shunmue!
http://www.youtube.com/watch?v=F3S34YqHHQY
http://www.youtube.com/watch?v=jrNCP6XuL9A
Feel the beautiful sadness!!!

Frank Klepacki: C&C games
Jeremy Soule: Total Annhilation, KotOR, CoH, and SupCom
Mark Griskey: KotOR II
RealX: EVE Online (Although gets annoying after you've heard it over and over and over...)
Gregory Alper and Jeehun Hwang: Mechwarrior 2 Series
Special mention for the BF1942 theme. Sometimes I just can't get it out of my head, hehe.

Almost forgot, another Shenmue song of beautiful emoness!
http://www.youtube.com/watch?v=ZTHtgKiQxY

I'm not usually the one to pay close attention to soundtracks but every GTA game I played had a nice collection of music.
Unless we're talking about original soundtracks in which case it will have to be Fallout 2.

/Me looks through his music collection and tries to think of what he listens most do (Yes, I actually buy game soundtracks on CD and online)
From most to least often played (only OST)
Chrono Cross
Command and Conquer
Final Fantasy VIII
Outlaws
Silent Hill (collection of the best songs from 1-4)
Alice
SonicR
Dino Crisis
Incubation
There's also a long list of individual songs, but these are the albums.

Battletoads soundtrack was kick ass....especially when the game was paused...

Oooh, yes, Battletoads and Secret of Mana: I would take out the cartridge on the title screen and the music would play forever.

FYI: This is a pretty great site for listening to a lot of great video game soundtracks.
http://www.gamemusicjukebox.com/

First of all: Independence War 2: Edge of Chaos. It is one of the best soundtracks I've ever heard. Even better: it is included as MP3s in the installation folders.
Others:
Chrono Trigger
Chrono Cross
System Shock
Willow (NES)
Solstice (NES)
Homeworld. Samuel Barber's Agnus Day sung by a choir is wonderful and moving, very well chosen for Chapter 3.
Myst / Riven
Outcast
Flashback
Shadow of the Colossus, great interaction with the game.
Alone in the Dark (2008), although it becomes a bit repetitive if listened out of the game. It uses some nice tricks to improve the general feeling of the game.
I forgot Omikron: The Nomad Soul! Unforgivable. A soundtrack made out of original Bowie songs plus compositions by Reeves Gabrels? That, my friends, cannot be topped.
And now that I mention Quantic Dream, I must say that Angelo Badalamenti made a good job in Fahrenheit: Indigo Prophecy. It was really tension filled and worked very well. I really hope these French will hire another great musician for Heavy Rain. And also that, for the first time, they will manage to keep the same title in Europe and USA!

I was looking forward to Heavy Rain, then I found out it was PS3 exclusive.

Shenmue
jet set radio
donkey kong country
starfox
metal gear solid
world of warcraft
halo
phantasy star online
oblivion
bioshock
return fire
sonic cd
streets of rage 2
soo many to list

Max Payne
Max Payne 2
Everything composed by Nobuo Uematsu
Silent Hill
Silent Hill 2
Silent Hill 3
Silent Hill 4
Fallout
Fallout 2
Painkiller
Portal
Half-Life series
Wow... there's way too many to list...
Vagrant Story
Chrono Trigger
Chrono Cross
LittleBigPlanet
BioShock
System Shock
Okay... yeah... I could go on forever

Hey, here's the Arcanum main menu theme.
http://www.youtube.com/watch?v=ftvpi-6Z6qw

so after replaying grim fandango i must say music there was so awesome.
loved every single tune.