Ultima Weapon

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.

FF6 Overworld in-gameFF2 DoS
-Ultima Weapon
FF6 Overworld in-game FF6 Atma Weapon in-gameFF6
-Atma (NA) / Ultima Buster
-Atma Weapon (NA) / Ultima Weapon (JP)
FF7 in-game FF7 in-gameFF7
-Ultimate Weapon (NA) / Ultima Weapon (JP)
FF8 in-gameFF8
-Ultima Weapon
FF10 in-gameFF10
-Ultima Weapon
FFX-2
-Ultima Weapon
FF11 in-gameFF11
-Ultima