This is down to personal preference and usage.
Personally i'd have to say Ultima, though Zodiark is incredibly strong when used properly.
zodiark
Belias,Matues,Adremmelech, Framfrit,Exodus,Shehamzai, Zodiark,Ultima,Hasmal, Cuchulainn,Chaos,Zalera, Zeromus.
weakness is holy
its in a treasure chest
Ultima Weapon.
1 hit from knights of the round will do it
vip for papaboy100
yes i forget how many though
One of the recipes can be used for making the Ultima Weapon.
Tidus's Celestial weapon, or Ultima is the caladbolg
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Renzokuken does 250000, omnislash does 150000.