The only ones I've really ever dealt with are Ultima and Omega in FFVIII. Oh, I do think I beat a superboss or two in X-2 as well. I had a lot of fun with those just because I loved the battle system. I honestly can't remember anything about them, though.

Ultima was probably harder in VIII, but I had all of my characters leveled to 100 and didn't protect against death the one time I went against Omega. He used LVL5 Death (or whatever it was called) and killed everyone. I just decided it wasn't worth it in that moment.