Suggested by Destroyer Sephiroth is one of the most iconic villains of all time. His skills with a blade are nearly unmatched and it’s hard to keep him down. In a straight fight there are few who can challenge him, but Aizen has no need to engage him like that. He can use his illusions to make the match more difficult and in his final Xenomorph-esque form he has more speed and power than Sephiroth. When you put all of those factors together then it really is the end for the one winged angel. Aizen wins.