The Ultima Weapon is the most well-known optional boss in Final Fantasy. In FF6, Atma Weapon (Ultima Weapon (JP)) is a mandatory boss on the Floating Continent, and not as powerful. Its brother Atma (Ultima Buster (JP)) lives in Kefka's Tower and is optional. In FF7, the Ultima Weapon takes a different form as a flying beast who keeps running away after battle. In FF8, the Ultima Weapon lives in the bottom of the Deep Sea Research Center and is the owner of the GF Eden. His weapon looks suspiciously like Cloud's Ultima Weapon sword. FF10's Ultima Weapon is actually purportedly a "shadow" of Omega Weapon and resides in the Omega Ruins. In FFX-2, it is just a randomly-encountered monster.
-Atma (NA) / Ultima Buster
-Atma Weapon (NA) / Ultima Weapon (JP)
-Ultimate Weapon (NA) / Ultima Weapon (JP)