Harry Greb was nobody's chump, maybe the best ever, right up there with Robinson. Marciano is underrated, sure he was small and easy to hit like everybody said but all he did was beat everybody. Archie Moore was older for sure but he peaked later in his career in my opinion as did Walcott. Marciano had heart and guts like very few have had. You can't take the HUMAN aspect out of it ever, Ali on paper should have lost to Liston and Foreman and handled Norton no problem, but it was not the case.
Dempsey was an absolute killer but partied too much. Jack Johnson was a defensive wizard, could bang with anybody in any era. You have to consider modern training methods with old time fighters when considering ''who was the best"- put the rock, Greb, Johnson, Dempsey under modern training methods and they are all bigger ,faster, stronger etc..
It's like saying the '27 Yankees and Murderer's row was the greatest team ever- Ruth, Gherig , Lazzeri , Combs, etc... but they would lose to a modern team because of training methods, skill development, better pitching today etc... Bring them to modern times and train them then maybe ....