And why (list some tracks while you're at it)?
My choice for best OST is FFX's soundtrack and I honestly don't think it's even a contest.
First of all, the sound quality is better than the games that came before it, so that makes a big difference. In FFX, you're getting a far "fuller" sound than its predecessors. However, it's not sound quality alone that makes it the best. After all, XII and XIII have better sound quality, but their soundtracks are far more forgettable.
The main reason I think FFX's soundtrack is the best is because it's exotic. I credit this to the fact that it wasn't just Nobou Uematsu working on the music for this game. There were three composers and as a result there seems to be more sonic variety on display here. Not to mention that the locales had more of a South East Asian feel to them, which had to be worked into the music. The previous FF's had a more European atmosphere to them.
I also think that FFX's soundtrack has more sonic texture. There is more atmosphere and ambience than in previous soundtracks, but the melodies are still memorable, unlike FFXIII, which felt like it erred too much on the side of ambience.
As for some examples of the best I think the soundtrack has to offer:
Someday The Dream Will End. This lush orchestration of one of the game's main musical motifs is stunning. It starts out really gentle and dreamy, but it builds up to the point that the dreamy melody floats above a more more forceful, monolithic sound that matches the sort of ancient largeness of the Zanarkand ruins.
Besaid Island. Another really lush track whose purpose is to communicate the feel of a lush island paradise. They could have just went with some generic island music, but this track is really interesting. Listen to the bells in the background, that pulsating synth sound in the back (which kind of reminds me of the rhythm of the tides going in and out), the interesting use of snare drum. All of this combines to form something really peaceful but also exotic.
The Sight of Spira. This is a really gorgeous acoustic track that really gives the villages in Spira a really lived-in feel. The ambient and rather "cavernous" use of aucoustic guitar is really brilliant and again, there's something exotic about this track. The periodic knocking on wood and the somewhat discordant part where the guitarist seems to "mess up" intentionally really make the song interesting and gives it a bit of grittiness, which matches the sort of low-tech lifestyle of Besaid and Kilika.
Wandering Flame. This is probably the most beautiful "ambient" track that I've heard in an RPG. It gives a distinct feeling of loneliness and mystery, but there's a really subtle jazzy undercurrent that's irresistable.
Blitzball Gamblers. This track is really moving because it feels like pure triumph. You can feel the adrenaline in the drums, horns, and strings. It's a really bright, triumphant track, but it's not at all cheesy.
Silence Before The Storm. One of the most atmopsheric songs on the OST. It has a really pleasant sound to it, but for all its dreamy beauty, there is something really mysterious about it. Something that kind of reminds one of unsettling creatures in a forest labyrinth. Another song that gives me this same feeling is Saria's Song from LOZ: Ocarina of Time.
Thunder Plains. This one has a kind of hypnotic quality to it, starting from the metronome in the beginning. The music is alot more gentle than you would think it would be for a setting based on thunder, but the choppy, rolling piano still communicates that feel in a rather unorthodox way. This track is a really good example of what I mean by exotic. There's a certain strangeness to this track that makes it stand out.
Luca. This song has alot of what I love about the Besaid Island song, but there's a more frantic sound to it that really suits Luca's hectic mercantile atmosphere. There is sort of a seafaring undercurrent to it that I really like. You can imagine people carrying stuff through crowded streets.
Phantoms. This song has crystalline quality to it that sounds like ice and snow and what not. But at the same time, it has the sort of driving monolithicness to it that Someday the Dream Will End does. This song really reminds me of that one in many ways, except this one has a more arctic feel to it and isn't as melodic.
I love the main theme (To Zanarkand) as well, but I feel its kind of played out and tho it has a moving melody, I feel it's not quite as interesting as the tracks I've listed, probably because it's not as dense.
There are tons more songs I wanted to list, so my post is only scratching the surface, but this post is all ready too long as it is.
I know I've used the word exotic too much, but it really describes FFX's OST very well. There's a more feminine feel to this game's music than in the other games, but there's alot of grit to the music as well. It really gives off a mysterious, lush, and ancient feeling that is evident in so much of the game's visual design.
Also, the vocal track in FFX is alot less cheesy than Melodies of Life and Eyes on Me from FFIX and FFVIII. I still like those songs alot because they have strong melodies, but they are pretty cheesy. Suteki da ne is not really cheesy at all. I think it's a Japanese folk song, tho I could be wrong.
Anyway, I've talked way too much. What is your favorite soundtrack and why?