Okay, of the ones I've played a decent amount of:
IV: "Mystic Mysidia," "The Red Wings"
VI: "Celes's Theme"
VII: I love a lot of this soundtrack, though my current favorites are "The Temple of the Ancients," "Judgement Day," and "Wutai Ruins."
VIII: I also quite enjoy this soundtrack. My favorites are "Silence and Motion," "Compression of Time," and "The Mission."
IX: "You are not Alone"
X: Many favorites, though my current favorites are "Calm before the Storm," "Summoned Beast Battle," and "Someday the Dream Will End"
X-2: "Moonflow/Mi'ihen Highroad," "Eternity," and "Zanarkand."
XII: I ABSOLUTELY LOVE "Ozmone Plains," "Esper Battle," and "Eruyt Village."