Ozma, because his battle, no matter how strong and powerful you are, will always require some luck in order to beat him.
Proto-Babil (FF IV)
Omega Mk. II (FF V)
Neo Shinryu (FF V)
Kaiser Dragon (FF VI)
Emerald Weapon (FF VII)
Ruby Weapon (FF VII)
Omega Weapon (FF VIII)
Ozma (FF IX)
Nemesis (FF X)
Penance (FF X)
Paragon + Trema (FF X-2)
Chac (FF X-2)
Major Numerus (FF X-2)
Yiazmat (FF XII)
Omega Mk. XII (FF XII)
The Judge Magisters (FF XII)
Vercingetorix (FF XIII)
Long Gui (FF XIII)
Gilgamesh (FF XIII-2)
Valfodr (FF XIII-2)