Ultima Weapon and Excalibur are always known to be the best. While Katanas have an excellent combo rate they rarely enable combos in Final Fantasy games. In a single attack Excalibur wins with it's also trade mark of auto-haste and holy elemental.

Another great damage dealing is Lances. These weapons compare to damage and status boosts as well. Paired with Jump and these weapons can easily hit max damage. One some occasions the spear is greater then the sword but not always.

I still have to go with Great Swords though cause of basic damage.