The most powerful weapon Sora can obtain in Kingdom Hearts is Ultima Weapon. It can be made at the Moogles' Synth Shop in Traverse Town, and shows up on the list only when all the other items on the Synth Shop list have been made at least once. Once it becomes available, these are the items you'll need and which creature drops them:

Lucid Gems - Wight Knight (Halloween Town)
5 Power Gems - Pirate and Pirate Ship (Never Land)
5 Thunder Gems - Aquatank and Screwdriver (Atlantica)
3 Mystery Goos - White Mushroom, Black Fungus and Rare Truffle (various locations)
3 Gales - Angel Star and Invisible (End of the World)