user14329: 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.)

user4418: There has almost certainly never been a "world's best player" in the last few hundred years who would lose to some of the weak CMs around today (or even in 2020). You do not need a rating of 2200 to be a CM in today's world. You can be sub 1200 and "win" the title by finishing in the first 3 in a continental championship for under 8's.
To see a list of weak CMs go to the FIDE advanced ratings search page and search for CMs according to rating ascending. There are currently 7 players rated under 1200 in the list of CMs.

user-1: 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

14329: <p>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?</p>
<p>(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.)</p>
 4418: <p>There has almost certainly never been a &quot;world's best player&quot; in the last few hundred years who would lose to some of the weak CMs around today (or even in 2020). You do not need a rating of 2200 to be a CM in today's world. You can be sub 1200 and &quot;win&quot; the title by finishing in the first 3 in a continental championship for under 8's.</p>
<p>To see a list of weak CMs go to the <a href="https://ratings.fide.com/advaction.phtml?idcode=&amp;name=&amp;title=c&amp;other_title=&amp;country=%25&amp;sex=&amp;srating=0&amp;erating=3000&amp;birthday=&amp;radio=rating&amp;line=asc" rel="nofollow noreferrer">FIDE advanced ratings search page</a> and search for CMs according to rating ascending. There are currently 7 players rated under 1200 in the list of CMs.</p>
 -1: <p>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</p>
