Since this topic comes up every couple of hours, I'll do something different with my list...I'll pick soundtracks that work perfectly on their own, with no nostalgia involved(buh bye chiptunes). Albums that are just filled with damn well composed and played music, not just nostalgic video game music with catchy tunes and themes(buh bye 99.9% of Nintendo's games). Note: I love the music of Nintendo's top tier franchises.

Also, these be original soundtracks, not seperately released arranged albums.
1.-
Xenosaga (Sleigh Bells edition, only Disc I) - Yasunori Mitsuda
An
epic space operatic masterpiece, this is Mitsuda's best and most complex work to date(ditch the rose-tinted glasses, CT's OST is not even on the same galaxy). With the London Philharmonic Orchestra lending their talented players and the Metro Voices lending their vocal chords, no other game soundtrack even approaches this on a technical level. Soundtrack has a dark, somber tone...fitting for the space theme I guess.

Just excellent stuff, worthy of a listen even if you don't care/give a shit about the game(personally, I barely do).
Personal favorite: "Omega". Just fucking badass.
Note: DISC 1 of the reprint edition of the soundtrack(released by YM's label) contains all the *real* tracks of the soundtrack(the stuff played by the London Philharmonic), the track order is orders of magnitude better than the one in the original release. DISC 2 is entirely forgettable and discardable as all the in-game music(read: synth-based) was dumped on it, rendering entirely worthless.
2.-
Shadow of the Colossus - Kô Ôtani
I had already fallen in love with this soundtrack literally more than a year before the game was released, ever since the first video was leaked I was lovin' Ôtani's work. I went as far as making mp3s out of the videos released by game sites, and personally ripped the music from the american website. In short, I had listened to most of the soundtrack before the game was even released...just crazy.
Majestic is the word to describe this soundtrack, the themes work perfectly and make you feel as if you're in the middle of battle, even if you've never even touched the game. Like Xenosaga, Kô Ôtani uses real instruments to drive his compositions. The ending themes in particular are very powerful.
Personal favorite: "Counterattack". This bitch hits all the right notes exactly when it has to.
3.-
Soukaigi - Hiroki Kikuta
I'd say that this OST is freaking
energetic. I don't know SHIT about the game, never played it, I'm not even sure if I ever saw a picture of it. I pretty much stumbled upon a couple of tracks from the album by accident some years ago. Kikuta uses a mix of high-end synths and real instruments here. It's pretty different from my other two picks, the music in here is much more eclectic in nature, some of the tracks being downright ethnic, with a bit of middle eastern influence. The guitar-based tracks are particularly bitchin', with plenty of nice playing. I don't understand how Kikuta isn't getting any jobs in the industry, what the fuck is wrong with game companies.
Personal favorite: "Fire Wire". Rockin'.