"Warriors of Light": In canon, the FF1 Warrior of Light is just one guy. In fact, the FF3 "Onion Knight" was just one guy until the revamp gave the four distinct looks and backstories (which weren't really much). I would say the only thing that makes either "character" powerful is the fact that "Cosmos" (aka the crystals) selected them to be the hero of the story.

Cosmos and Chaos: Well, they are gods in the Final Fantasy multi-verse (although how a god can have a beginning is a bit perplexing), making their place on the list rather obvious.

Shinryu and Omega: Considering both are dimension-transcending entities, one created to destroy the other, each almost always serving as an optional encounter protecting or leading to an ultimate item or the story's end, there's very little question to why they are on the list.

The Final Bosses (not the extra ones randomly appearing afterward): Palamecia, Xande, Zeromus, Exdeath, Kefka, Sephiroth, Ultimecia, Kuja, Yu Yevon, Vayne Solidor, and Orphan... well, I mean, come on. Final boss. (insomuch as the story allows them to make sense... seriously... introducing Cloud of Darkness and Necron as final bosses when we'd had no previous reference to them..? Come on.)

Noah/Doga/Unei: Apart from Xande, clearly, the most powerful beings in Final Fantasy III (Leviathan and Bahamut not withstanding), Noah having been the magus that taught Xande and made him mortal. Noah was even able to seal away (and later summon) Leviathan and Bahamut.

Cloud of Darkness: A dweller of the World of Darkness, it has the desire and likely enough the means to reduce the world to nothingness, a trait shared by Necron of Final Fantasy IX (either another creature of the World of Darkness or an alternate incarnation of the Cloud of Darkness itself). Definitely powerful.

The Lunarians (Cecil, Theodore/Golbez, FuSoYa, Zemus): Implied as being comparable to supermen next to the Earth-dwellers. Fair? Not really.

Enuo: A wizard who traded immortality to rule the Void, he wrought havoc until his defeat. Scary.

Terra: The hybrid daughter of Maduin, the Guardian of the Espers, it is Terra's birthright to be insanely powerful.

Genesis and Angeal: A kin to Sephiroth, Genesis could be seen as arguably more powerful than Sephiroth. While Sephiroth's genes spawned only four "working clones" (if you include Cloud), the term applying to the physical and mental capacity of said beings, Genesis's genes created far more easily recognizable clones, many going so far as to copy him down to his attire. Angeals abilities are even greater.

Cloud: If you observe the version from Kingdom Hearts, he possesses many abilities absent in Final Fantasy VII including a semblance of flight and various other powers spawned from darkness. If not for that, I would not submit him as a candidate for powerful entities of the multi-verse.

Vincent, Hojo, and Fuhito: Hojo experimented on everyone by combining Mako, Jenova material, and human DNA. The results created Sephiroths, SOLDIER, and Vincent's and Hojo's transformations. While Vincent can become four separate inhuman entities, only the demon that connects him to Garland, Chaos, really strikes a blow against the competition. Hojo's transformations are much more limited. His first being to far too greatly resemble Jenova's broken form. The other being his unnamed lifeform state: a floating torso with sharpened arms and a fairly scorpion-like tail. It may not seem all that impressive by looking at it, but when you account for the fact all of this came from his insane ingenuity, he's pretty scary. Shortly following Hojo's mutilation of the human genome via his experiments on his unborn child and the Turk Vincent, ultimate environmental terrorist Fuhito created the Ravens after studying SOLDIER and then implanted himself with the ultimate summoning materia Zirconiade.

Minerva/WEAPONs: Seemingly the physical representation of the Lifestream, Minerva seems to be an Ermac-type construct: a goddess composed of a multi-consciousness derived of the all the souls and spirit energy which form the Lifestream. While this potentially makes her more powerful than any of the WEAPONs, she is perhaps less dangerous due to having self-control. While the other seven known WEAPONs are meant only to rampage over the earth and destroy it (Omega's purpose being to harvest the resulting sudden surge of spirit energy and transplant it to a new planet a la Terra), Minerva's purpose is to express the Lifestream's usually unconscious will. Point of fact: these are the most powerful beings on the planet. The only reason Sephiroth (and by extension Genesis) was able to potentially surpass these beings is due to possessing DNA from an alien creature.

JENOVA: The Calamity from the Sky, an alien creature that left a crater large enough to swallow Midgar, possessing both potent magic and the ability to infect the locals with its will although modern science would have you believe Jenova has no sentience and thus no will.

Adel: A powerful sorceress and former ruler of Esthar, it is assumed that like Edea, Adel worked as a political accomplice before usurping control of Esthar in a brutal coup. It was her ambition to dominate the planet much like the Persian Emperor in 300 with whom she shares a giant stature (and muscular non-effeminate frame).

Ultimecia: Time-compression-what? Ultimecia frequently possesses the bodies of others sorceress and can also manifest ideas from people's minds and junction those realized ideas to her own body (tada! It's Griever!).

Zidane: He and Kuja both possess alien abilities surpassing the skills of normal humans in physical ability and in, Kuja's case, even magical prowess. They're Terran form is restored when they enter Trance state and Kuja is even able to fly.

Necron: A creature that appears following Kuja's defeat spouting the purpose of life is to end and its apparent desire to reduce all to naught. Gee, does this sound familiar?

Seymour Guado: Seymour begins as a fairly typical summoner and skilled mage despite being a Guado-human hybrid. However, following his death, he becomes a powerful and malignant unsent due to Yuna being denied the ability to perform a sending.

Yunalesca: The first summoner to defeat Sin and daughter of Yu Yevon, she developed the method by which a summoner would bring about the Calm: the Final Aeon created from the bond between a guardian and the summoner. As an unsent, she became even more dangerous.

Barthandelus: Self-proclaimed Lord-Sovereign Barthandelus possesses great magic and the ability to shapeshift into other forms.

Master Xehanort/Xemnas/Ansem: The false Ansem, the self-proclaimed Seeker of Darkness, Ansem has the ability to utilize darkness to influence others, possess those whose hearts are open to darkness, summon Heartless, and to manifest as a gigantic and powerful creature. Xemnas, the Superior of the In-Between, is the wielder of nothingness thus granting him the ability to perform attacks that are greatly similar to some used by his Organization. Blades of energy, surges of electricity, flight, fields of lasers, illusions... he's fierce. Master Xehanort, the original form of Ansem/Xemnas and Terra-Xehanort, his magic is arguably off-the-scale, able to raise the dirt at his feet into a 60-foot tall cliff at a lazy gesture and even control keyblades telekinetically.

Sora/Roxas/Xion: The light-based keybladers, Sora can take on alternate forms, Roxas can wield two keyblades (a trait later passed to Sora), Xion can fire magic from her bare-hands.

Riku/Repliku/Riku-Ansem: Riku walks the path "to dawn", a direction that allows him to edge on darkness while remaining a warrior of light. Repliku and Riku-Ansem both have the ability to utilize dark aura, a power running contrary to the keyblade's natural tendency toward light.

Ventus/Vanitas: Ventus has the ability to fire beams of light and produce swords of light that resemble wings. Vanitas possesses extreme versatility in physical and magical combat and his depth of control over darkness allows him to wield a powerful beam of darkness.

Lavos/Queen Zeal: An alien parasite with the ability to imitate other powerful creatures. It has a unique relationship with sorceress Queen Zeal as it obliges her wishes and does not attempt to absorb her into itself despite her immense power being a potential boon.

Magus: Janus takes after his mother, being among the most powerful beings alive with devastating magic and physical abilities.