Sephiroth's source of power is Jenova AND the Lifestream. Lifestream is the source of all magic and life in the world of FFVII, and Sephiroth actually suceeded in controlling it, in AC.
He managed to give Geostigma to some children. If he really managed to control all of Lifestream, Cloud wouldn't have been able to stop him, and Seph would have already won. But I would agree that he controlled maybe at least half of Lifestream, but not all.

Well, this is only your opinion, and I will give my: I would say that if we give prep to them Kefka could win, but in a direct combat Sephiroth would be the winner.
Kefka does have more power than Sephiroth, but yeah, Seph would definitely rip Kefka to shreds with his sword. After all, power doesn't necessarily determines who would win a fight.

Avalanche are much better fighters than the Returners. AC and DoC proves that.
Isn't this also an opinion as well, since you have yet to actually explain what this "proof" is?

And I don't know about DoC, but Avalanche didn't really seem to do much in AC. Avalanche beat a few monsters. Cloud was the one who actually defeated Kadaj, Yazoo, Loz, Bahamut, and Sephiroth.


And Kefka having the higher level of power is just opinion. In my opinion, using the corrupted version of the source of magic in the world, to transform the planet in a dead vessel and use it to travell the universe, is much more impressive than destroying some cities with a light from the sky.
And terrorizing the world for many years is more impressive than infecting a few children with Geostigma.