Ultimecia wasn't hard at all, this coming from someone that almost never grinds in JRPGs. I'm not saying it was easy, but I don't remember having any trouble at all. Ultima Weapon in FF8, now that was a boss to remember, one of the most epic fights I've ever had.
But on topic, the hardest I remember is Wiegraf in FFT. I thought I was so screwed since I had saved, luckily I managed with some luck and some job changes to beat him








