Ali was very fond of Johnson's for political reasons. I don't think Ali is being honest. A low guard, low punch out fighter with a short reach being Ali's toughest fight? Nope, nope, nope. Ali did gain respect for Marciano when the two sparred 60+ 1 minute rounds for the computer fight movie. The story goes that Ali knocked off Marciano's toupee, and Rocky became angry and landed some hard body shots. Johnson could match up with past greats before him or up to Marciano, but its pretty much game over for him post Liston.
Johnson gets too many passes in and out of the ring. Holmes was 48-0 at one point. Besides the version of Spinks who fought Holmes would have been Johnson's best title opponent. Those who watch film and read primary source papers know Johnson was a very uneven performer, and prone to draw or upset with lesser talents. There is not one all time great heavyweight who gets this many passes. Let the ring records, film, and print rule. In this case Johnson was Ko'd by a supper middle, boxed even by a past his best middle, and floored by a another middle. He drew with a joruneyman in a title bout, and face men who today would not be top 50 in other title fights. No ATG heavy has this many dubious results on his resume, and no ATG heavy ducked the top competition as champion as often as Johnson did. We have been over this ad nausea, and none of the Johnson crotch jockeys will take you up on known facts when the truth is not on their side.