Originally Posted by
Sephiroth with liberties by Yours Truly
Sora is extremely powerful. He gets stronger as his heart becomes stronger... through experience and his friends. It may be that not only his heart collects experience, but that is the real source of power in the universe of Kingdom Hearts.
Sora's problem: he can only use his own potential when wielding a Keyblade, a weapon which is proof enough that his heart is strong. No FF character or any non-Keyblade KH character shares that problem. They all have enough other ability to fight, whether psychic, magic etc. and Sora, no matter his stats, is no all-powerful deity in the storyline. Donald is definitely stronger in terms of magic than Sora.
No matter what people think, not only Scar becomes a Heartless. Maleficent's Dragon form was unleashed by Key to Hearts the same as Sora's Shadow therefore is not only a powerful form she transforms into like she does outside of the Castle of Slumber, it is also her Heartless form. While Jafar, Ursula, et cetera didn't become traditional heartless they are also deeply steeped in darkness.
"Kingdom Hearts" is the powerful omnipresent source of all hearts in the Kingdom Hearts universe. But comparing it to God "whom" people believe to be the omnipotent, omniscient and omnipresent creator of everything is just... wrong. Especially the "Absorb God and you become God" theory. Not to mention that Xemnas' Kingdom Hearts was artificial and damaged by Ansem the Wise. And there is nothing more to say about that.
Xemnas is the most powerful Kingdom Hearts character. And the story proves that Sora alone is not able to defeat him. So the question of Sora being the most powerful Kingdom Hearts character is answered already. And I highly doubt that Sora would even stand a chance against the reborn version of Xehanort. That's the final battle of this Kingdom Hearts Saga and writing a main villain who can be defeated by the main character only is not going to happen. That's not Final Fantasy-like. And Kingdom Hearts is the child of Final Fantasy.
VILLAINS
Lord Garland/Chaos:
Garland creates a time loop and uses the power of the orbs to gain the power of Chaos, and Dissidia even shows that it is the natural power of Chaos, a power which is a old and great as the void, so he is not only a monster called "Chaos", even though that was definitely the meaning 25 years ago.
Emperor Mateus Paramecia:
Mateus reigns an empire, teaches the meaning of fear is manipulative and rules the afterlife as Lucifer himself.
Zande:
Zande freezes time and summons a Dark Cloud which should reset everything
Zemus:
Zemus controls minds even though he is sealed away and can return by nothing but hatred once he is dead.
Lord Exdeath:
Is a fusion of evil souls possessing a tree who is almost immortal so he had to be sealed away and even within his seal he was able to command his servants to destroy the crystals. He is an incredibly strong Black Mage who can create illusions, so he let the Otherworld crystals look like monsters so that Bartz and the others destroy them and open the Rift. He can transform into a part of a tree or a tree himself and even though his will was not strong enough he controlled the power of the void for some time. And the his true power, the power of Neo X-Death, so the fusion of the evil souls which were sealed in the forest and which are released and controlled by the void in the end is incredible.
Commander Kefka Palazzo:
Is extremely manipulative and so insane you never know what his next step is. Even though he plans to restore the statues and absorbs some Esper's power before he absorbed their power as a spontaneous action when Celes hurt him with a sword. Then he devastates the world, builds a giant tower of trash and makes the land infertile with his Light of Judgment.
Sephiroth:
Has a stronger will than everyone else and an indestructable one. He can control JENOVA cells and also use them to regenerate and reunite. He can absorb life energy, will power and knowledge, everything which is the source of the true power in Final Fantasy VII and since he resists the lifestream he became so strong that he doesn't need Materia for magic if the knowledge of how to use it is within the lifestream. He can create illusions which look and feell as if they were real and since he has the strongest and most powerful will of all he can easily fool everyone. He has the ability to shapeshift, he has psychic abilities like levitating/flying, telekineses and telepathy and reading minds and memories, he can teleport, he can go through matter, et cetera. He is the strongest wizard, no wonder with the strongest will which can get stronger limitless and he is the strongest swordsman and the one one who can use Masamune like he does and not only with the Iai-Style since it is a very long and heavy Nodachi. And he can destroy the solar system, no matter what people think. Stats = Nothing, Scene: Demonstration of Story Abilities.
AND: Before Advent Children he even reassured not to fade by linking his existence with Cloud's memories.
Ultimecia:
She can control the power of other Ultimecia's one they die in another time period to control other witches in another time period and so take a look at that time period with her conciousness. She is an incredibly powerful Time Mage. She can read minds and make them real.
Baron Kuja "King":
He is a Weapon Seller who manipulates a queen, darkens her heart and finally absorbs it to get into Trance mode. He destroy a planet. Then he "just" wants to destroy the Existence Crystal. But, no, no way he is powerful. He is just a Super Saiyan 4.
Maester Seymour Guado:
Very manipulative and a genius in terms of strategies. Physically pushes back a I don't know how many tons-heavy [Sin] Spawn. Wants to destroy all lifeforms to release them and he even does destroy many of them. And most of it as an Unsent One.
Emperor Vayne Cardias Solidor:
Like Seymour very manipulative and a genius in terms of strategies. He also wants to make things easier by destroying them and teams with Venat to destroy the Undying Ones. He has a super airship, the Bahamut and uses manufacted Nethicite. And then he fuses with Venat to become an Undying One himself who is able to destroy a whole airship just with a beam of his Aura.
Galenth Dysley:
Manipulates the heroes and forces them to go through a whole Final Fantasy. Strong enough to kill them all. Nothing more to say.
Like the true Sephiroth says to Sora:
"I admit you're very skilled."
And that's it. No more and no less. Only the dirt Sephiroth wipes off of his leather coat once he is done with Sora.