Best Music Poll

A fair warning: I'm a composer by profession, so this post is going to be a bit lengthy. tl;dr at the bottom.

While I absolutely hate everything else about the game, VIII is Uematsu's most mature and musically significant work. He uses a lot of sophisticated techniques and compositional styles in the soundtrack, and it's pretty impressive considering the vast majority of his music in every other game is, well, the opposite really (though I will say I really hate J-Pop and have never been able to forgive VIII for forcing it on us once in several FFs after this).

But VI gets my vote, because while overall VIII's soundtrack is technically better and more mature, it's also overall more boring, while most of VI's soundtrack is entertaining the whole way through. VI is also the highpoint in Uematsu's FF soundtracks as far as leitmotif goes (the use of themes for characters and locations), in that he expertly weaves themes in and out of other pieces and even other themes. "Dancing Mad" is an excellent example of this. While many people are aware of the various themes from the game meshed into the piece, almost everyone seems to miss all the subtle quotations of other themes. Nearly every character's theme is quoted at some point in the first three movements, though almost all of them are hardly noticeable. This is fitting though, considering there is no real main character of the game. I think the only ones that aren't quoted are Relm's, Gogo's and Umaro's themes. I can't remember off the top of my head, but Strago and Mog's themes might also be missing. Of course, most of the themes aren't recognizable to most people because the whole piece is mostly a variation on "Kefka's Theme", with a little bit of the "Opening Theme" thrown in too. Mixing other character themes, which are mostly of a lighter nature, into the sinister and dark variation on Kefka's Theme is obviously not going to make them stand out too much.

And let's not forget the "Ending Theme", with it's outstanding variations on Setzer's theme and beautiful takes on Terra's Theme.

Anyway, as a quick summary of others:

FFI: Have to respect it for the Prelude (although that version is just the scales and is quite boring) and the Final Fantasy theme. Otherwise it's a very immature and subpar effort (yes, even considering some other classic songs from the game).
II: A major improvement. Uematsu seemed to really start taking the composition role a bit more seriously at this time, whether that's true or not I don't know, but through the music it seems that way. As an aside, the "Main Theme" is one of my favorite pieces from a video game. It's too bad it doesn't get much attention or remixes.
III: Uematsu's dark side really shows in this one. And by that I don't mean "evil" or anything like that, just that the music is very atmospheric and moody. This is an excellent example of using the NES' capabilities for sound to its fullest.
IV: A favorite of mine for its nostalgic value, though there are a lot of duds in this soundtrack. I think I remember in an interview him saying it was due to the game as a whole being rushed, and him not having much time to get used to the new capabilities of the SNES and such. All things considered he did an excellent job.
V: Lots of mixed feelings on this one. There are some pieces that are absolutely beautiful. Then there are others that are just dreadful. There was a lot of potential here, but ultimately I think Uematsu missed his mark here. A more experienced composer could have done great things with some of these themes.
VII: Much like the game itself, completely overrated. Still good, but not as good as it should have been. Uematsu hit his stride in VI, and he should have kept the momentum going into VII, but he didn't. A lot of the pieces just don't make sense musically, the instrumentation is questionable and about half of the soundtrack is just forgettable. Yet because of three songs ("Aeris' Theme", "Main Theme", "One-Winged Angel"), most people act like this is a masterpiece. As good as those are, and that's debatable, that doesn't make up for the rest of the soundtrack. Like I said, good, but not great.
IX: My second favorite game in the series, yet at the same time, due to the nature of the game it hurts the soundtrack a bit. IX went for a less mature feeling than VII and VIII (not in a bad way, just a more fun and cartoonish way), and the soundtrack had to reflect that. Not to mention that since IX was an homage to the other games in the series, the soundtrack had to be as well, and Uematsu thus reused a lot of music from previous games. If IX's soundtrack didn't include that stupid J-Pop song, I think the soundtrack could have been better overall, not having to rely on quoting it in every other piece, but oh well.
X: LOL awful