The ali from '66 beats anybody. At that point in his career his speed and movement were at their absolute peak. And he had a chin so it's not a matter of catching him with a lucky punch, it's a matter of consistantly connecting and nobody throughout the history of the heavyweight division had the necessary package of speed (both with punches and ring movement), skill, or stamina (many had any two of the three but never all three) to keep enough pressure on him to beat him.
I got a dvd for christmas which lists the top 10 heavyweights of all time according to burt sugar (a very good espn ringside dvd with old fight footage for dempsey and even jack johnson). Holmes was 10th which made me an instant fan of the dvd. He gave louis the nod over ali and when confronted with larry's (who was present so they could tell him he was number 10

) view point of louis would never catch ali, burt said that head to head matchups were not part of his criteria. And he put 25 title defenses (22 by ko) ahead of ali's resume. He also admitted he may have had a bias seeing as louis was his hero growing up.
So if you're comparing in a head-to-head mtach up to determine hypothetical greatness, ali has to be the favorite but if you want to compare greatness on the scale of just what each guy did in his era, there's room for debate there.
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