In Kingdom Hearts, and in other role playing games, there exists usually an ultimate or legendary weapon that has the strongest traits above all weapons, such is the case in Kingdom Hearts. For your three member party of Sora, Donald and Goofy; they each have their own ultimate weapon, in the form of their weapon usage of choice. Obviously, each weapon has prerequisites that entail several seemingly daunting tasks that you must undertake to obtain these Ultimate Weapons.

Sora's Ultima Weapon
The Ultima Weapon is no doubt Sora's Ultimate Weapon, taking it's roots from the Square's Final Fantasy series. The Ultima Weapon has all the good factors of all previous other keyblades with virtually no bad impact to Sora. To obtain this weapon, you must Synthesize certain and different materials to create the Weapon. (See Synthesis Sidequest Section). The items are easy to locate, and they are located in all around the worlds.

Donald's Save The Queen
Another Final Fantasy weapon import, Save The Queen is a staff in Kingdom Hearts, like the Ultima Weapon, has all the good impact stats to increase Donald's statistics overall. Save The Queen is obtained from completing the Hades Cup, only Sora. Which means, your party is only Sora while you face the Hades Cup.

Goofy's Save The King
Again, Goofy's Save The King is his Ultimate Weapon, Goofy's Ultimate Weapon literaly has no downgrading effect to his stats like the other party members. To obtain the weapon, requires you to complete the Hades Cup Time Trial, which the time limit is 20 Minutes.