All of them! ...That's kind of a cop-out, isn't it? But the boss music almost always ranks among my favorite tracks in a Final Fantasy game. Specifically, though Jenova's theme is pretty cool. Nice and spacy. The music to Ultimecia's last form is highly rockin', and though I haven't played FFX, I've heard the last boss music from that one too and like it quite a bit. I can just picture the story situation that would result in music like that, without even playing the game at all. Cool stuff.