Well, The heroes seem to be very balanced compared to each other.
But I think Terra might have an advantage here, since she's half Esper and was able to burn 50 soldiers in a very short amount of time. Next would be Zidane, due to his Trance ability.
Other than that, the others seem to be at the same level.
As for the villains, I usually rank them by tiers:
Tier 1:
Exdeath
Ultimecia
Chaos
Reasons:
I consider Exdeath to be the most powerful ff villain to date, because of his control of the Void and, later, his fusion with it, giving birth to Neo Exdeath and granting him the power to destroy all existence.
Ultimecia comes right next to Exdeath, because she can control time to an extent and, after achieving the Time Compression, she was able to absorb all existence into herself.
Chaos, in the original Final Fantasy, didn’t seem very powerful, being just a demon possessing the combined powers of Lich, Tiamat, Kraken and Marilith.
But if Chaos was originally meant to be the almighty God of Dischord that he was in Dissidia, then he would be one of the most powerfull FF villains, along with Exdeath and Ultimecia
Tier 2:
Kuja, Sephiroth, Kefka and Sin.
Reasons:
Kuja gained enormous power after absorbing the souls in the Invicible and achieving his own Trance.
His Trance ability, Ultima, was able to leave Terra largely destroyed in a short amount of time.
In terms of raw destructive power, I would put him a little above the other villains in this tier, due to not only Ultima’s power, but also due to its speed.
Sephiroth possess the most powerful spirit/will in the planet, allowing him to go beyond the laws of Nature. He was also able to corrupt the Lifestream itself and create his own, achieving to an extent, what he wanted to achieve in FFVII.
Though we’re familiar with his physical prowess, we don’t know Sephiroth’s true power, but if the Negative Lifestream is similar in power to the portion of Lifestream that Aerith used to destroy Meteor, then Sephiroth would possess enough power to destroy a small planetoid like Meteor. Using this power against the Planet would probably cause a enormous wound on it, 2 to 3 times bigger than the Northern Crater.
Nevertheless, the Negative Lifestream can be used in many different ways. Sephiroth was using it to corrupt the normal Lifestream and corrode away the planet, and he used it to erect a barrier around Midgar so that no one could interfere with his battle against Cloud. There’s also the Shadow Creepers, who were spawned from the Negative Lifestream.
Kefka was able to absorb the power of the Warring Triad, becoming the God of Magic and providing him enough power to leave the entire planet in ruins.
His Light of Judgement was very powerful, being able to destroy entire towns and leave scars in the planet itself.
But being the God of Magic, Kefka also had all the magic in the world in his hands, and he was able to erect an enormous tower to serve as his base. He probably also created all the monsters that populate Kefka’s tower.
Ultimately, while Kuja has the upper hand in raw destructive power, Sephiroth and Kefka have the upper hand in the tenacity and versatility of their powers (Kefka due to his status as the God of Magic and Sephiroth due to his spirit/will and his own version of the Lifestream). All in all, they're equals.
Finally, we have Sin, a gigantic beast whose Giga Graviton scarred the planet in such a way that the damage was visible from outer space.
Level 3:
Emperor Mateus Palamecia
Vayne
Zemus
Reasons:
Emperor Mateus has a spirit/will that rivals Sephiroth’s. After his death, he was able to take control of Hell and Heaven (taking in consideration Soul of Rebirth) and return much more powerful than before. He possesses enormous magical power.
Vayne gained a lot of power after fusing himself with manufacted nethicite and with Venat, being able to destroy several airships with little effort-
Zemus is another being that possess a very powerful spirit, so powerful in fact that, after his death, his spirit formed a new very powerful being, Zeromus.
Level 4:
Golbez
Garland
Reasons:
Golbez is a very powerful mage, possessing high level spells and very strong physically, being able to survive almost uscathed from a Meteor spell.
Garland is a very strong knight, maybe stronger physically than most heroes and villains.
Since I haven’t played FFIII yet, I decided to leave Cloud of Darkness out of the list, mainly because I don’t know her abilities, besides the Particle Beam/Wave Cannon and that fact that she’s an embodiment of the Void/Nothingness.
