Most recent "best in world" player who would be defeated by a CM Of the chess players who could reasonably be considered the best in the world in at least one year in history, how far back would you have to go before one of them would lose to a 2020 Candidate Master (or FM or IM) in a classical game? How would historical figures such as Steinitz or McDonnell or Ruy López fare against the different ranks of titled players?
(Going back far enough, mileage on rules like castling or stalemate may vary. If you think it makes a difference which player's version of rules is used then give the answer both ways.)
McDonnell was never best in the world, and I doubt he would have been stronger than 2200 FIDE, a candidate master would be an even match. Out of the best players in the world, both Philidor and Labourdonnais would both be capable of being defeated by a CM, with regularity. It is difficult to compare strengths of the players from the past, because they would be possibly strong GM level in tactical calculation, but weak in openings and modern strategy. But I would estimate a regular 2200 FIDE CM could beat them on occasion