It's hard to say. Each game had great music, so picking a favorite is difficult.
I'd have to say PSIII, though. I love most of the music in it, and the theme itself is very moving.

I also like Wren's theme, Skyhaven, Shusoran's theme before you enter the castle, LaShute, and Dark Force's theme. I could keep going, but I'd probably list most of the music in the game...
Out of the other games, I loved Palma's theme in PSI, Violation in PSII, and a handful of tracks from PSIV. Laughter, Thray, and Dezoris 2 are just a few.
