For my money, it's FFVII. Certainly the best full soundtrack on my Sansa. Uematsu is divine.
However, let it be known that Yoko Shimomura's "Another Side" from Kingdom Hearts: Final Mix, tops the entire thing. The problem is that with all the Disney-oriented msuic she has to write for the KH series, she has yet to compose a whole collection of pieces better as a whole than the FFVII soundtrack.
However I've seen the trailer for Versus XIII and. . .wow. Just amazing. For Opera style vocal tracks, I'd have to say that's the best, and yes, I did consider "Aria de Mezzo Cattare". It looks like this is her moment.
Of course, I look forward to the whole game as well. The premise sounds great, there's hope for multi-dimensional characters, a bleak setting, and the gameplay looks like it will be a much better, much more difficult Kingdom Hearts, in other words, everything Kingdom Hearts should be at this point.
I anticipate Versus XIII to be, frankly, one of the best games Square has ever made, and the best Final Fantasy spinoff to boot. It could possibly become the best game to bear the name at all, like Tactics once was before we all realized that, yes, FVII still existed, and before Final Fantasy IX came out.