The Ultimate Weapon also known as "Darkness' Opposite" or "The Final Sword" is a sword rumored to be the first Keyblade, it's the strongest blade in the game, yet third longest, it was created by Ultima. It appears in all games.

Ultimate Weapon
480px-Χ-blade (Complete) KHBBS
Ultimate Weapon
Vital statistics
Type Blade of Light
Effects +12 Strength, +12 Magic, +7 Length.
Source La'Kana's ship.
Cost to buy 1,000,000 Gil, 5 Meteorites, 10 Angelic Stones, 5 Mythic Crystals, All Arichalcum+'s(Normal Arichalcum's in GCIV)
Cost to sell 500,000 Gil
Ultimate weapon vs omega weapon

The "Ultimate Weapon" and the "Omega Weapon (Vanitas' Blade)"