The hardest I've come across so far is Ex-Death, the friggen tree. Who knew? Still haven't beat him. Runner up for that: Altima. She's...

Easiest? Chaos on FF1, part of Origins. Because Haste on a level 35 BlackBelt and his attacks. That's why. Runner up on that one: Sephiroth. Because damn, if you can beat him at level 25, he's too easy.

The most fun boss fight I've ever had, though, is Kefka hands down. Him, his statues, his dungeon, all of it. Hard as hell, puzzling, and just plain dizzy... yeah, Kefka!!! YAAAY! ^_^ The ending was worth it, of course. Runner up on most fun? Kuja and his guardians. Then Necron. Maaan, that Necron...