They're both awesome; a tie. Personally, I think if they did come across one and other, Sephiroth would say Ansem was unworthy; Ansem would say Sephiroth wasn't worth the effort. Seph would brandish his sword, and Ansem would laugh, Seph would loose it, and attack. Guardian shows up, blocks Sephy, and Ansem mocks him. Sephiroth teleports behind him and attacks. Ansem gets mad, and starts his Strike Raid-like move and Seph blocks what he can. Sephiroth calls Meteor and attacks Ansem. He is hit, a lot, so he calls upon the Realm Hearts and Darkness. Sephiroth makes good use of his wing and performs jumps and acrobatics while attacking. Ansem blocks and attacks back with his halberd. They battle extrodinarily in the Realm of Darkness until it ends. Personally, if one goes down, I think, he's taking the other with him.