Oh no! Don't make me choose!! Please, Oh God, Why?!
Ahem... anyway, although I'm torn by having to make this kind of decision, I would have to say...
Hm.... Melodies Of Life, preferably the Japanese version but also the english and intrumental and the 'Layers of Harmony version (they're all so great ^^) I love the english in this situation, though, because I can sing it. Well, not so much when other people are around. But the tempo of it is just perfect for a theme that finds itself in nearly a dozen different songs throughout the game. The composition of the music is breath-taking. The vocals are extreamly well translated to english and they sound so rapturous and smooth to the ear. I guess this is why I have about 6 or 7 versions of this piece. Great stuff from Uematsu, and in 4 days (today is the 15th) I will be at the concert in Chicago, I can't wait '^.^'.
