Most of the Final Boss is not the hardest, well I point out some of the boss that are more insane than the final boss:

FFVII: Ruby Weapon (700000HP), Emerald Weapon (1000000HP)
FFVIII: Omega Weapon (850000HP)
FFIX: Ozma (64355HP, if I'm correct)
FFX: Nemesis (9999999HP), Penance (12000000HP)