Omega Weapon while a relatively easy superboss was also a rather fun one for me. If I look at it through my game design lens it's a pretty poorly designed one, basically going from "Level 5 Gatekeep which you just have to know about" to "doing basically no damage" to "suddenly almost TPKing your party" to "doing basically no damage", rinse and repeat. But probably the most satisfying thing about the fight is that he actually has the HP to withstand a full-on assault for a while and allow you to let loose with all the Lion Hearts and Quick Shots and Zell Duels and whatever else and not just have the battle end immediately for it. God that felt good.

But yeah, the FF series has few truly tough optional bosses, and they're all mostly in the PS2 era.