The Final Fantasy series has presented dozens of mystical worlds, and in each universe there is a form of magic. So, we've been introduced to many ridiculously heavy-hitting spellcasters over the years. To level the playing field, this list will exclude born-deities and mindless monsters. Though, this won't exclude mages who were originally people and later became gods, spirits, or fiends (assuming they're still sentient afterward).
To simplify, we'll be classifying these mages as anyone that can cast a black magic spell from the series (Firaga, Meteor, Flare, whatever). Well arguably, any characters above a certain benchmark (like level 50) are overpowered to their world's denizens. Theoretically many could destroy civilizations; varying in intensity from the systematic eradication of key areas to the scale of annihilating a whole planet with one spell.
One might ask, why didn't any heroes make this list? Perhaps a protagonist's strength is dependent on the playthrough, where some gamers might play without using mages. This list will show some of the canonically most powerful mages in each main entry in the series. It will also very loosely attempt to order them into the lowest, middle, and highest five of destructive potential. Be prepared for spoilers and read on!
15 Seymour Guado (Final Fantasy X)
Let's start with someone familiar to most of you. Seymour was a powerful summoner and sorcerer. When you first fight him, his elemental spells are already stronger than when he was temporarily playable (and he is able to multicast). Every time he is killed, he comes back even stronger — for a total of four forms. In only his third, he is one of the hardest non-optional bosses in the series, and nearly brings the Ronso to extinction.
His final plan was to pilot Sin from the inside, and save the people of Spira from the pain of 'life.' Well, with the Final Summoning abolished, succeeding would have placed him higher (if it wasn't an insane idea).
14 Vayne Carudas Solidor (Final Fantasy XII)
Is anyone else playing FFXII: Zodiac Age? If you don't know him, he masterminds the invasions of nations in the game to obtain nethicite. He kills anyone in the way of him taking the throne (including his brothers, his father, and the Senate). When the player faces him, he fights like a Monk would. But with the power of the nethicite, he transforms into Vayne Novus.
Vayne is beastly, and becomes able to utilize all elemental magic, while also negating magical attacks against him. After defeating him again, his Occuria friend, Venat, fuses with him, imbuing him with more power (and doubling his health). He utilizes this to fuse mechanical components of the Sky Fortress onto his body, becoming the Undying. In this form, all of his spells pierce through Reflect.
13 Lahabrea (Final Fantasy XIV)
Picking someone to represent FFXIV was tough, as many of the final bosses and super bosses were armored warriors or inhuman beings. But here is Lahabrea, the Overlord of the Ascians; a cult intending to revive their god, Zodiark. He is a powerful spellcaster, and is capable of calling monsters from the Void, teleporting, and possession. Admittedly, he is considered an easy final boss at the end of the campaign.
Lahabrea appears again as the penultimate boss in the Heavensward expansion. He is notably fiercer, and also fighting alongside Igeyorhm. Halfway through their health, the two fuse together, becoming Ascian Prime. In this form, he utilizes dangerous dark magic, inflicts many status effects, and will instantly kill the entire party with Annihilation if they do not destroy either his orb of fire or ice.
12 Paddra Ballad-Caius (Final Fantasy XIII-2)
His name means Caius of the Ballad clan, from the city of Paddra, and he is a tragic figure from the storyline. He was a Pulse l'Cie, and guarded the seeress Yeul whose visions would kill her. He was given immortality, and every time Yeul died, she reincarnated. This cycle of watching her die drives him mad over the centuries.
If I didn't have a 'villains' theme here, I probably would have put Lightning here, to be honest; by the end of the trilogy, she's stronger than gods. And too many other contenders were never human. As well, Caius utilizes more physical skills (and buffs) than most of the others here. However, he does rely very heavily on his Graviton spell in battle, and occasionally uses Inferno as well. He can summon meteors, command chaos to create time paradoxes, and summon (and become) Bahamut.
11 Lady Lilith (Final Fantasy XI)
Lady Lilith was from the original timeline; a dark reality where the Shadow Lord never lost. After forging a pact with Odin, Lilith defeats the Shadow Lord. But when the Goddess Atlana decided that this chronology was too sad, she replaces it with a nicer one. To defend herself and her history from being erased, Lilith tries to negate the birth her alternate self. When she fails, she absorbs her other self instead to become a goddess, Lilith Ascendant.
There were other options for FFXI. Eald'narche almost destroyed the world, and had control over the Shadow Lord. But the Shadow Lord himself (the dead spirit of Raogrimm) was my first choice as the first final boss; and later, in-battle as Arch Dynamis Lord, he is so strong that gamers had to complain (though everything Dynamis is a dream). Still, Lilith is no pushover, wielding strong magic like Meteor, Comet, and unique dark spells.
10 Garland (Final Fantasy I)
People really overestimate Chaos. He claims to be a god, but he's just another demonic monster. He was originally a Cornelian knight named Garland. After his defeat by the Warriors of Light, he is pulled back two thousand years into the past by the four manifestations of his immense hatred.
With the Dark Crystal, he unleashes his Four Fiends of Chaos throughout history to wreak havoc on various civilizations. When the Warriors of Light travel back in time to face him, Garland absorbs his fiends to assume his demonic form. He is capable of utilizing the monstrous elemental abilities from his fiends: Tsunami, Cyclone, Earthquake, and Blaze; but the biggest threat is his Flare spell.
9 Xande (Final Fantasy III)
The Great Magus Noah had three students. He bestows a gift to each; vast knowledge of magic, absolute control of the world of dreams, and to Xande; mortality. Driven mad at the prospect of dying, the enraged Xande drains two of the Crystals, disrupting the balance of the world. A flood of darkness covers the surface of the planet, freezing time for everyone except the denizens of a small floating continent, and allowing Xande to never age.
Well, Xande could have been placed as the representative of FFXIV, but he was the main antagonist of FFIII first. When fought, Xande is a master of elemental magic, and also utilizes Meteor. When the heroes defeat him, it is revealed that he was being manipulated by the Cloud of Darkness, who revels in Xande absorbing all of Crystals. The Cloud doesn't quite fit the bill here of ever being human though.
8 Zemus (Final Fantasy IV)
Despite the later placements in the strongest category, this isn't the roster of the Dissidia villains, and Golbez doesn't compare to Zemus. With his immense psychic powers, even while sealed away, he was able to mind-control Golbez for the majority of FFIV. With the Giant of Babil, Zemus could have ended all life on the planet, if not for heroes.
Zemus taunts the released Golbez into killing him. But death only makes Zemus stronger, as he comes back as an embodiment of hate, Zeromus. In this form, he is incorporeal, unable to be harmed, yet capable of causing destruction with Meteor. It is only with the power of the Crystal, and prayers of hope that Zeromus' form is revealed. In the final battle, he can use Flare, Maelstrom, Black Hole, and his signature move, Big Bang.
7 Ardyn Izunia (Final Fantasy XV)
I hope you played FFXV already, cause here come the spoilers...! Two thousand years before the beginning of the game, Ardyn was a Lucian king who tried to save his world from the Starscourge, a curse that turned people into daemons. He absorbs the entirety of the blight, becoming immortal but tainted. Rejected by his bloodline and by the afterlife, he begins a lengthy plot to plunge the world into darkness.
Admittedly, his plans do take years to destroy civilization, but he has all the time in the world. After getting his opposition out of the way, Ardyn unleashes the Starscourge, and daemons overrun the planet for ten years. When Noctis returns, Ardyn is fought as the final boss, capable of warping around, using elemental magic, and utilizing darkness in his attacks.
6 Emperor Mateus (Final Fantasy II)
Mateus forges a dark pact in order to summon an army of demons into the world. The Emperor's magic is powerful enough to lift a castle, or destroy entire towns with tornadoes. And, of course, all of this is before he is killed. Afterward, in Hell, part of the Emperor's soul kills the devil and claims his throne, amplifying his own powers. He brings Hell to Earth, and becomes intent on destroying the world rather than conquering it.
The other half of the Emperor's soul takes over Heaven. As his hellish half can only be destroyed after his counterpart is killed, the party's comrades who had died have to take down the Light Emperor as well. The only other wizard in the game who might compare is Deumion, the immortal guardian of the ultimate magic, and the game's optional super-boss.
5 Enuo (Final Fantasy V)
Don't forget, Exdeath is just a tree that was used for sealing malevolent souls — he was never really a person. But Enuo was the inspiration for creating such a prison in the first place. Okay, so Exdeath can create disasters to take down entire forests and islands. He even multiplies in power when he channels some of the Void, able to erase entire areas out of existence. In his last boss form, Exdeath becomes capable of destroying the universe, but in the process, he loses his sanity.
Meanwhile, a thousand years before the game, Enuo traded his immortality to create and have complete control over the Void. He had conquered his planet before being defeated by the twelve legendary weapons. Then, he was consumed by the Void. By seeking out Enuo's soul there, the player finds the game's optional super-boss. And Enuo's Essence is notably stronger than the final Exdeath battle.
4 Kuja (Final Fantasy IX)
Kuja doesn't get much credit, but he's a bigger threat than people realize. Kuja is capable of fashioning magical beings using the Mist, as well as commanding the wills of monsters including dragons. He was created by Garland to cause as much death as possible on Gaia, so he genocides the summoners and begins inciting wars between the nations.
Kuja eventually betrays Garland. In Kuja's first time entering Trance, his magical powers increase exponentially. He destroys their home planet of Terra by use of Ultima, and turns to Gaia next. He loses the final battle against the heroes, but unleashes Ultima again, to finish killing everyone. In a Purgatorial plane, Zidane must defeat the god of death in order to annul the end of the world and the end of all life.
3 Sephiroth (Final Fantasy VII)
Sephiroth was already superhuman from birth, but after assimilating with Jenova and manipulating the Lifestream, he becomes exponentially stronger. It is implied that whenever he is defeated in an installment of the series, he wasn't able to fight to his full potential. Sephiroth is more well-known for his swordsmanship, but he is also a master of elemental magic.
Considering the lore of ancillary games, perhaps Genesis after absorbing the Goddess materia could have fit here. Or Vincent might have worked, as the Protomateria encases Chaos, whose sole purpose is to kill all life on the planet. But when Sephiroth acquires the Black Materia, he gains Meteor, capable of destroying the planet. And this doesn't consider the few planets he destroys with his limit break, Supernova.
2 Kefka Palazzo (Final Fantasy VI)
Kefka is an ultimate evil that few can be compared to. At nineteen years old, he was given magical power; but as the experimental process wasn't perfected yet, he went mad during the infusion. He is fought several times throughout the story, typically using elemental magic, Drain, and Poison. Outside of battle, he is capable of projecting illusions, including one of himself to fight for him.
Kefka eventually upsets the balance between the Warring Triad of gods. This causes the end of the world, and Kefka absorbs all of the power from the triad. He becomes the god of magic; ruling the world from atop his tower by smiting any town that defies him off the face of the planet. It's only after the heroes return to oppose him that Kefka decides to just destroy everything, but if he hadn't basked in his power for a year, then he would have succeeded in this.
1 Ultimecia (Final Fantasy VIII)
Ultimecia is a sorceress from the future; an era where she has already taken over the world. However, she is aware of her destiny to be defeated by a legendary warrior. To avoid this, she decides to compress all of space and time. By doing this, she will be able to become "a living god" and absorb all of existence. She sends her mind back in time to cast the time compression spell.
Ultimecia is capable of giving life to inanimate objects and even concepts. She has telekinesis and can control minds. In battle, expect her to use Quake, Meltdown, and Bio, but as she builds power, she utilizes Ultima, Meteor, and Flare. If not for Squall stopping her before she finishes her spell, Ultimecia would have taken not only the planet out of reality, but also everything in the universe.