There isn't a single ultimate weapon, but my list of suggested items is Black Hole, Wizard Staff, Enemy Fighter Jet, Magic (Blue ball that isolates the first object it touches), Pick, Bazooka,...
In both games, you have to synthesize it. In Kingdom Hearts, you need: * 5 Lucid Gem * 5 Power Gem * 5 Thunder Gem * 3 Mystery Goo * 3 Gale In Kingdom Hearts 2, you need: * 13 Orichalcum+ * 1...