If we're talking regular battle music; FFVI.
If we're talking about all the battle themes in each game, FFVI still gets my vote. Every one of the battle themes was amazing in that game.

I must say, I'm kind of surprised how many people have chosen FFVIII. Man with the Machine Gun was good, as was the final boss theme, but the others bored me to tears, and those two still don't compare to the themes in FFVI if you ask me.