Li Grim

Final Fantasy: Tactics Advance

The personification of all the dreams and the dreamers. Kind of a downer to beat her, actually. She can change laws at whim, and deal pretty big damage to multiple units. Other than that, she isn't much of a challenge.

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Light Emperor

Final Fantasy 2: Dawn of Souls

The true form of Emperor Palamecia. Her soul is good, but lives in Hell.

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Major Numerus

Final Fantasy X-2: International + Last Mission

A large hydra fought in the coliseum.

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Minerva

Crisis Core: Final Fantasy VII

The boss in the final mission, with a raft of incredible attacks. She also appears in the ending as the "goddess of the planet".

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The final boss of FF9 is more symbolic than relevant to the storyline (See FF9 Characters. Its signature attack Grand Cross (used also by FF5's final boss) causes many status ailments and lots of damage. Once you figure out how to defeat it, the fight's over pretty quickly.

Also known as "Omega Gold". This optional boss is created at the monster arena, after you've captured ten of every single monster in Spira and beaten every other monster in the Monster Arena. He resembles a palette swap of Ultima Weapon. I personally haven't encountered it yet, but they say he's one mother of a fight!

Exdeath the "tree" uses a very unsettling "Void" attack which just makes one character disappear! After he is defeated he transforms into a multi-headed, crazy boss, possibly the hardest final boss in the entire series. It uses a variety of status problems, physical attacks, magic attacks, defensive spells, and the almighty Almagest spell. Winner of the Most Psychedelic Background award. See also FF5 Characters.

A very bizarre optional boss, whose battle takes place at the Chocobo Air Garden. Ozma is by far the hardest boss of FF9 (far eclipsing Necron). It resembles nothing more than a colorful sphere. It gets to move about three times a round, and consistently cures itself or casts powerful magic (including ones that cause big status problems or kill instantly). What's worse, unless you pursue yet another side quest, you can't use physical attacks against it! When you win you get a useless Strategy Guide and a very useful Pumice and Dark Matter.

This guy just keeps coming back for more, even after he dies! Still, a bit more wimpy than most final bosses. See also FF2 Characters.

Penance

Final Fantasy 10

This boss is only seen in FF10 International. Lacking that version, I don't know much else about it, but it's apparently even harder than the Dark Aeons.

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Promathia

Final Fantasy 11

The final boss of the FF11 "Chains of Promathia" expansion pack. He is the twilight god, on par with Altana. He created the ark angels and the beastmen. He cursed people of Vana'diel for attempting to open the gates of paradise. This curse is the emptiness threatening to consume the land. To complete his purpose, he must be resurrected and killed so that every citizen will return to him when he dies.

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Like Emerald Weapon, this special boss was only in FF7 (NA) and the Japanese re-release, FF7 International. It lives under the Corel Desert (once it appears), with only a tiny red arm sticking out, waiting for a curious player to come investigate. It has the ability to eliminate two players from battle, leaving only one, and it cannot be defeated unless its two claws are buried in the sand. If you defeat it, a man in Kalm Town will give you a free Gold Chocobo!