there's no way louis would have been able to keep any version of tyson from running him over, he was too stationary and his chin wasn't sturdy enough, and he never fought anyone with bone breaking power and speed at the same time. but that's a different topic though.
it can be difficult to define a good defense, because theres so much too it. Tyson had superb defense at his best incorporating a full spectrum of skills including head movement, foowork, arms and hands and if you missed him (which you would) he'd throw precise, devastating counters to head and body, and his positioning was always deadly. his counter threat was so consistently effective that many opponents chose not to punch at all as the fights went on, as to punch was often suicide. id rate him as number one as he combined great refelexes with serious skills, not even tried, let alone mastered, by many. Id rate joe Louis high too. yes he was caught at times but then he had such a long reign. Louis, like Tyson was an excellent boxer who could force the opponent to punch then unload with brutal counters. So many of louis' opponents were kayoed as they came in punching only to have the perfectly positioned Louis waiting for them with sledgehammer counters. ali had great upper body movement and footwork at his best and was usually in a great position to counter. I also think larry holes had great defense based around footwork, a great jab and rhythm and he was a master of parrying and smothering and blocking using his arms. very rare technique reminiscent of jack johnson
James J. Corbett, Jack Johnson, Gene Tunney, Jersey Joe Walcott, Muhammad Ali, Jimmy Young, Chris Byrd, Fat James Toney, Mike Tyson (for offensive fighters), and Packey McFarland (at his true weight).
In a prime for prime scenario that's very debatable, especially when neither fighter has faced anyone quite like the other. Louis is very dangerous going backwards as is Tyson going forwards. I'm not here to argue about this fight, but for what it's worth I think Tyson has a slightly greater chance of victory. Regsrdless neither Louis or Holyfield would make it near my top 20 let alone top 10
maybe a shout for Mike the bounty Hunter. When he choose to go strictly defense he was real tough to hit.
I'd probably say Toney - if we're talking pure technical skill. Haven't watched a lot of Byrd, so I can't say much about him. Walcott . very, very nice to watch, but his low hands and tactical decisions cost him in two crucial fights. Young and Tyson - both very good, but also a bit overrated. As for Young, a defensive master just to doesn't stick his upper body through the ropes to escape the tepid attack of the worst version of Ali to that date. Tyson's was very inconsistent in his head movement. Even before Douglas he'd have long stretches in fights where he didn't move his head much. Frazier is probably a bit underrated. Contrary to Tyson, he actually did have extremely consistent head movement in his prime. He was always there to be hit in the first round, but that was because he needed a round or two to find his rhythm. Holyfield had a very technically sound defense, but not those lightning fast reflexes. The skill level of Ali's defense often gets underrated, because of some of the basic thing he did contrary to the text book. His tactical smartness (when to engage, when to cover up, when to move, when to hold etc) was a big part of his defense.
Chris Byrd, James Toney, and Jimmy Young deserve a mention. I'd add Ali, Holmes, Lewis, V. and W. Klitschko, Tyson, and Johnson