see while Sephiroth was a strong warrior as part of project S, and even at his strongest he could use super Nova, he still isn't the strongest villain in the final fantasy franchise,
Kefka was a GOD while Sepha-Sephiroth was just a arch angel in comparison, Kekfa ruled over magic and creation in his story using both those powers to destroy in a beam as slowly as he wished to make people suffer,
plus if we are going to look at other villains from the franchise Ultimecia is stronger then sephiroth due to her being able to compress time and space, even one of her forms quite literally has the fabric of reality going into her,
then you have Bhenivelze a god of creation that created 3 gods under him, governing life and death creating planets, and chaos even with his death he took planets and destroyed them into nothingness, and even created life to take over once they gained souls.
plus you cannot ignore both Chaos and garland from ff1 and the emperor from ff2 who fought eternaly (for chaos) and ruled over both heaven and hell (emperor)
but ignoring the villains then you have the heroes, Cloud beat sephiroth multiple times, Lightning defeted a god, terra killed a god a lot of the heroes are far more powerful then sephiroth, and seeing how omega is fascinated by the anomaly of heroes winning, it wouldn't shock me if we fight THE warrior of light and other heroes in the alpha scape before Omega.
if anything i predict we will fight all previous heroes in one battle from ff1 to ff13 (and maybe ff15) in a nice homage to the final fantasy heroes in one tier of the alphascape, with other tiers being for super bosses like emerald weapon, penance and finally Omega.
