Umaro was awesome. Easily the best physical attacker in FFVI. Not having control over him was actually a boon, since you didn't lose time on choosing commands. He just wrecked trout up. And it was magical.

Sadly, while I love to use him, I think Kimahri wins as the most useless. It's really unfair that his Sphere Grid - both the standard and expert version - is so in the middle of it all that you really have to put in tons of effort to actually make him worthwhile.