The Jeopardy Ultimate Tournament of Champions was the show's way of giving the retired champs a make good on their 5 game cap reversal, but short of inviting all of them back to play regular season games ala Blockbusters, there weren't a lot of ways to equitably solve that problem.
So, let's do the next best thing- speculate. If we dropped all of the retired five game champs into post-season 20 Jeopardy, who's pulling off the longest win streak? Make the assumption that we're dropping them off at the age they were originally on the show.