Ozma of 9, though... *runs screaming in terror*
I had beaten Ozma without doing the sidequest. (mostly because I didn't know about it the first time I played through...
) And after about NINE ATTEMPTS I *finally* defeated ozma. Although I often found myself *screaming* "SON OF A B****" and promptly chucking the controller across the room.
.
What? No love for Omega & Shinryu from 5? Or maybe even Intangir from 6 (w/o X-Zone)?
I beat all of them on the first try, albeit with suped up characters (on 5 anyways, if you're not at least capable of defeating the main boss and then some, "don't even try until you get better or stronger", whichever comes first), they weren't easy by any means, just nowhere near as difficult as normal Ozma or *limiter released* Indalecio from Star Ocean 2.
... wait, you can not use X-Zone on that boss? Wow, that never occured to me before 
Tonberry King was the hardest boss in the FF series, I think. One touch kills and there really isn't much of a way to level up enough to make him easier.
Tonberry King from FF8? I didn't really find him extremely difficult. Of course I never really gave him the chance to get *too* dangerous.
