I always liked Kiros' (FFVIII) limit break, Blood Pain.
It's not especially powerful, and you have maybe two opportunities to use it throughout the entire game, but it looks incredibly stylish.
I also really like all of FFVI's Desperation Attacks, mostly because I didn't even know they existed until recently. Apparently, if you use the "Fight" command with a character in critical health status, there's a 1 in 16 chance they'll use a very powerful unique skill. It's basically a precursor to the limit break as we know it today.