Not so much soundtrack, but one of the best users of sound as something integral to gameplay was the Thief series.

Soundtrack wise: WCII and WoW both have amazing soundtracks.
I don't get why so many people are so keen on the Halo soundtracks. They're good, sure, but not amazing in any way.

Also, can the song at the end of Portal count?

EDIT: How the hell did I forget Zelda?

For me it would defiantly have to be Castle Crashers, i can't get enough of the music from that.

I think you're all just depressed because you haven't thought to say that Banjo Kazooie truly had the best soundtrack of all time. I mean especially considering it was one of the few instances where game makers before the Xbox/PS2/GCube era thought that using real instruments was a GOOD idea.

Best use of xylophone ever.

Any Sonic game's

I haven't heard a GREAT soundtrack in years. Chrono Trigger, Xenogears, and FF 6+7 had amazing soundtracks. Halo 3 was pretty well done, nothing spectacular though. Fable 2 is pretty good too. Fallout 3 is pretty interesting, with all of its old-timey songs.

VCPR was awesome!!

Armored Core Last Raven was one of favourite as well

BioShock, man it's epic and disturbing at the same time.

LucanDesmond:
I haven't heard a GREAT soundtrack in years. Chrono Trigger, Xenogears, and FF 6+7 had amazing soundtracks. Halo 3 was pretty well done, nothing spectacular though. Fable 2 is pretty good too. Fallout 3 is pretty interesting, with all of its old-timey songs.

Yeah, Chrono Trigger was an incredible score. I also liked the OSTs from the Lunar games for SegaCD, brilliant games all around, imho.

World in Conflict has one of the best soundtracks I've heard in years; it nailed the cinematic side, and certain songs you can just listen to over and over again (Incursio, Bannon's Sacrifice, Remembering Sawyer) and they never get old.

Sim City 4 just had an abundance of incredible ambiance music, Deus Ex, Homeworld, Kingdom Hearts 2, Chrono Trigger was fantastic, the Max Payne theme song is all that game needed, Freespace 2, pretty much anything by Frank Klepacki; there's so much good gaming music out there.

Fable
Oblivion
GTA Vice City
Halo series
World of Warcraft

I loved GTA:IVs and DMC 4s but against the flow hated vice city's

morrowind
oblivion
final fantasy VII
SUPER MARIO
and other shit i can't remember

the halo soundtracks are good (only played halo: ce and halo 2 so i dunno bout 3) but its basically the same 2 themes presented differently.
For its sheer epicness and the fact that i listen to it on my ipod as well as wen im playing the game, it has to be the MGS4 soundtrack

Katamari Damacy, hands down.
World of Warcraft has some decent soundtracks.

Portals ending song - Still alive was awesome and easily the best ever ^^

Homeworld had some extremely well done stuff too.

Ive made a top 10 list of soundtracks for the xbox actually. http://www.gametrailers.com/player/usermovies/277438.html
Also, there are a lot of other games like these:

Rayman 2 had an amazing score. It was like something you could get out of a disney movie. All the songs were interactive so it changed when something happened, and fit each stage perfectly.

Age of conan had a very good soundtrack too.

Red steel had a surprisingly awesome score that nailed the japanese theme perfectly. Slow and sweet karaoke songs, crazy jap rock/pop songs, spooky ninja music that sounds like something from an old fighting movie, it was all there.

Other mentions go to the main theme of super mario galaxy, prince of persia Sot, halo 2, 007: everything or nothing, crash twinsanity, max payne, burnout 3, psychonauts, king kong, gta vice city, and SSX blur and on tour.

My personal favourite soundtrack is and always will be that from Morrowind. However, I've taken a liking to the '50s music of Fallout 3.

There are a lot I like, but curiously, one has come to mind

the original Sonic The Hedgehog

Parasite Eve, HAZE (its the only part of the game that lived to the hype :P).

I don't know if it's been mentioned yet. Jet Grind Radio and Jet Set Radio Future. Some of the most awesome music in gaming history.

GTA: Vice City takes the award from me.
However, that is not the only game worth mentioning.
I love the soundtracks to Oblivion, Shadow of the Colossus, and Ace Combat Zero.
If I was to make another prize, it would be for best original soundtrack (since Vice City doesn't have an original one), it would hands down go to Guild Wars: Eye of the North (Jeremy Soule is amazing with epic music).

MHunterZero:
I don't know if it's been mentioned yet. Jet Grind Radio and Jet Set Radio Future. Some of the most awesome music in gaming history.

Awesome and incredibly repetitive at the same time xD

Gormers1:
Ive made a top 10 list of soundtracks for the xbox actually. http://www.gametrailers.com/player/usermovies/277438.html

Watch and hear yourself.

Halo 2 and Halo 3 are the ones I listen to the most. Halo 2 has some awesome stuff with guitars, but overall doesn't have much depth. Halo 3 on the other hand... wow. The album is composed in a way that when you listen through it, it's like you're playing the game. The end pieces are the best ones... you really feel a sense of "it's all over". The Halo story is finally finished. It's an incredible soundtrack and easily gaming's best as far emotion and accomplishment goes.

Half Life series
The music and sometimes lack there-of makes the game realy immersive and awesomesauce

Shin Megami Tensei: Digital Devil Saga (Avatar Tuner).

The best soundtrack I have ever heard in a video game, but that's because I like guitar driven music in games. Shoji Meguro effectively conveys the meloncholy of the world through the area themes, the boss themes are adrenaline enducing rock anthems and even the little themes (like the shop theme and save point theme) are nice. The best thing about the music? It suits the each of the situations you are in, the End Boss music is some of the most epic music I've heard in video games.

Some of my favourite tunes;
Muladhara (Area theme, DDS1)
Sahasrara (Area theme, DDS1)
Big Battle (Regular Boss theme, DDS1)
Hunting (Regular Battle theme, DDS1)
Hunting-Compulsion (Mini Boss theme, DDS1)
Hunting-Comrades (Another Mini Boss theme, DDS1)
Hari-Hara The Second Movement (Final Boss theme, DDS1)

MODERN MASTERPIECE!
Ace combat 6

Fits the game perfectly, Acually all the music in this game rules.
Racedriver GRID

Just because I felt like adding one of those good old classics.
Daddly mulk, why the hell not?

If you get bored of the Ace Combat one jumt to 2.30 or something.

Breaking Benjamin and Incubus were awesome in Halo 2. I was hoping they'd be in Halo 3 :(

Of course Martin O'Donnell always kicks ass :D

For sheer comedy value I'd vote for the Katamari Damacy games :-D
For pure, awesome tunage I say the Wipeout 1 and 2 music (only Wipeouts I've played) and the tracks from Ridge Racer and Ridge Racer Revolution games.
Honourable mention also goes to the C&C series for Hell March :-P

I am so conflicted...

My money is on Castlevanias Symphony of the Night, Aria of Sorrow and Dawn of Sorrow (They borrow from the SotN so much), and Katamari Damacy.

And seriously, who can be mad when you hear those opening "Na na nana" of Katamari?

I'm surprised that no-one has said Super Mario Galaxy yet (at least I don't think anyone has) so it'll be mine.

Diablo, the Tristram music is just great, great, great.

Lost Odyssey had some great stuff in it as well.

the credit music in Mass Effect is great and was pretty unexpected, I was expecting some run-of-the-mill epic space-music and instead I got great music!

I absolutely loved the soundtrack in Braid. The music was not only great to begin with, but it gets better when you manipulate time, especially at the end when you're able to slow down time with the ring.

I'm a little obsessed with telling people how great this game is...

For me it has to be Golden Sun, with it's fast paced battle theme, haunting music of sol sanctum and the heroic theme of Venus Lighthouse it was incredible to listen to, the amazing thing was it was GBA audio and it still sounded epic.