Crisis Core: The Summoned
FFVII: Reunion (by far)
FFVIII: Ooooh, toughie...Maybe I'm a Lion, probably
FFIX: Melodies of Life
FFX: Gosh, there were only two tracks I didn't love to death! Final Battle and Summoned Monster Battle are tied for favorite!
FFX-2: Eternity and Demise, especially the piano versions
FFXII: Boss Battle