Ali and Johanssen never fought.

Another reason why Marciano was so well-regarded was that he was modest, articulate and well-spoken--a big contrast with the flashy, thuggish or inarticulate image of fighters in general. Patterson also was well-liked for his modest, even gentle, ways, and because he was an Olympian and was the youngest heavyweight champ ever. But the heavyweight division began going downhill after he ran out of credible opponents and was knocked down by some of them. After the the first Johanssen fight, a lot of people lost faith in Floyd--they thought his gentle manner indicated a lack of "killer instinct" that had sustained Dempsey and others. He regained some credibility after the Johanssen rematch (first-ever h'weight champ to regain the title), but the two Liston defeats sank him forever--he seemed paralyzed with fear. Liston's tenure marked the nadir of the h'weight division and boxing in general. Ali singlehandedly brought boxing back. Every fighter since owes him a debt.
Ali also was responsible for a landmark Supreme Court decision in the Civil Rights era:
The government hated Ali's "uppity" ways and the fact that he became a Muslim at a time of civil unrest and "Black Power." He was draft-eligible, but flunked the intelligence test ("I always said I was the greatest--not the smartest"). The Selective Service System then dumbed down the intelligence test--supposedly to meet higher draft calls for the Vietnam War, but really to ensnare Ali.
Ali then claimed exemption as a Muslim minister. His local draft board turned him down. Ali made numerous appeals, all turned down. Finally, he refused induction. He was immediately stripped of his championship, even though he hadn't even been indicted yet. He spent more than two years (and $2 million in legal fees) fighting it all the way to the Supreme Court.
The Court ruled, unanimously, in Ali's favor. He was registered in a Selective Service local district in Houston, TX. The population was nearly all black. But all of the Draft Board members were white. The Supreme Court ruled that, in the future, Draft Boards had to reflect the racial and ethic character of the neighborhoods they represented--a giant step forward in accountability, given the near-total power that the Selective Service had over young men at the time.

My favorite fighter (as opposed to the best fighter) was a Cuban welterweight, Kid Gavilan, who fought in the Forties and Fifties. I used to see him on the Friday Night Fights (with Jimmy Powers). His flashy style and "bolo" punch fascinated me as a kid. He had a 103-30 record with only 26 kayos. But he was the champ for several years, and he fought every welterweight contender of the era: Paddy DeMarco, Ray Robinson (twice), Ike Williams, Bobo Olson, Johnny Saxon, Johnny Bratton, Billy Graham, etc.