Tautology, you define a best debater as that.
So the most influential orators in history who had regular interlocutions with opponents and have managed to this day to persuade hundreds of millions to billions of people of positions despite their writings being thousands of years old is not a good tautology for a "best debater"?
Should we be looking at people who fail to convince anyone of things? Is this our metric? Isn't the purpose of a debate to have more convincing arguments and be more persuasive?
So why would the best debaters not be people who were successful at this?