Okay, I suppose I should have mentioned this; Super-Man is vulnerable to magic. He has absolutely no defense against it. He could easily be killed by a "Death" spell, put to "Sleep", "Petrified", transformed into "Mini" or a "Frog", and his speed could be cut in half by a "Slow" spell, or he could even be "Stopped" in his tracks. Even if he tried to pre-empt Sephiroth, Sephiroth would proabably have "Sneak Attack" equiped to his armor in conjuction with some other magic materia to combat Super-Man.
About 3000 Super-Man comics I own say I know what I am talking about.
In the comic series "Bat-Man Return of the Dark Knight", Bruce Wayne beat the living hell out of Super-Man, with the help of Green Arrow and his Mecca armor.
Bat-Man, people come on.
Did I forget to mention that Bruce Wayne was 65 years of age when he beat the hell out of Clark? He was. And Clark didn't even appear to have aged.
He just doesn't know how to use his awesome powers. He needs to go back to Super-Hero academy and retake Super-Powers 101, because he obviously received an F.
Even MARVEL's Thor has beaten Super-Man.
And if you saw Ultimate Avengers, Sephiroth moves a hell of a lot better than Thor in Advent Children.
You know why Thor beat Super-Man? Because Thor's powers are magic based.
Having said all that...I think that Sephiroth stands a significant chance.