It's really hard to compare, almost impossible some might say, villains through Final Fantasy franchise as we can't determine for sure how strong they really are, sure we can judge them by their accomplishments, but you have to understand that some villains had bigger opposition and harder time to accomplish their goal than other villains in series, who's there to say that Sephyroth wouldn't be able to destroy the world where Kefka resides as there's no lifestream to protect it (My God I'm defending Sephyroth, something's clearly wrong with me today).
As for Ultimecia, I like her as a villain, quite a mastermind, and bitch as Doodie would say, but that example you used, she pulling 3 planets and a large asteroid is somewhat stupid since it's just a flashy attack animation, it would be pretty stupid if she was actually capable of preforming such attack, which again would make Squall and his gang invincible for surviving it.
Same goes for Sephyroth's Supernova, if he was able to do something of such magnitude he wouldn't need a meteor, would he, though unfortunately there a quite a bunch of people who actually think he can pull that of. Storywise, Sephyroth is far from weak, he just didn't learned from his own mistakes and made few stupid decisions, but then again which villain hasn't done that as villain aren't supposed to win, with few exceptions "cough" Kefka"cough"Kuja"cough" (OMG, I'm defending Sephyroth again...weird...).
Bookmarks