Bahamut
Bahamut's Attack: Mega Flare
Location: Once you have the Ragnarok, you can go to the Deep Sea Research Center in the southwest of the World Map. Inside, you must answer Bahamut's questions correctly and then defeat Bahamut.
Useful Abilities: Abilityx4 makes getting this GF worth the effort. Rare Item is also nice to have if you're into collecting everything the game has. It's also nice to have a second Mug avalaible to build up your item stock.