Pffft easily the undying.

I dont count ff1 and 2 since gba versions are too easy.

Ffvi I can see someone dying. Vii I died. Viii is hard to get even to ulty if you cant junction.
X jecht is really powerful imo.

but undying is pish. Then again I was like lvl 60 and did all the hunts except yiazmat.