It's stated in the Reunion Files(official book released by SE) that AC Sephiroth is in his most powerful form ever.
But he didn't use his true power, because Sephiroth wanted to play with Cloud, not kill him in the first attack.

IMO Sephiroth wins. He can move with the insane speed of Chaos Vincent and Omega Weiss if he want(MUCH faster than any FF character until now), and his intangibility makes him immune to any attack Kefka throw against him.