Gotta go with Uematsu on this one. He's just been too prolific and varied in his composing career for me not to give him the nod. I'd give Sakimoto a good second place nod. I like pretty much every soundtrack he's done including FFXII which doesn't get enough love if you ask me. I also want to give a shout out to Jeremy Soule. Granted he only did one game for Square, but he's consistently impressed me over the last few years, and one game for Square is more than any other non-Japanese composer can claim to have done to my knowledge.

Mitsuda is a little hit and miss for me. Chrono Trigger was brilliant and Cross also had a lot of good stuff on it, but Xenosaga was an absolute bore. I can't remember everything else he's done, and haven't actually played Xenogears so it's difficult to judge if his missteps are less frequent than I perceive them to be, but in my experience he's not at the top of his game as often as Uematsu or Sakimoto.