Gilgamesh was one of the best parts of FFVIII especially because he kinda drops an FFV reference at the end of his super bad-ass sequence.

Mideen was also awesome for FFIX.

I'd like to see the arch fiends be made into this. I think the way they pulled it off with Gilgamesh shows that sub-bosses or optional ones like FFVII's weapons are optimal for this role.

It kinda makes me sad FF never really kept this tradition, there's so much room for it