Johnson's resume isn't as glittery as later heavyweight champ s but that's largely because of the era he fought in. He consistently beat the best opponents in front of him , including several hall of fame level guys. Your judging him by by modern standards with out considering the complex of the time s he fought in gives an incomplete picture.
I could judge him by 1930's standards and arrive at the same conclusion. Even with Dempsey''s egregious ducking of Wills I find his resume better. By Louis's time there is little comparison.
Johnson is rated highly by many due to his historical impact etc or as Emmanuel Steward pointed out due to his arrogance.
Does Dempsey have a better resume? Whatever idleness Johnson had as champion was forced upon him. Dempsey went Hollywood.
Dempsey had a much easier career as contender/champ. In fact, it couldn't be more drastic. Dempsey had the rails greased for him while Johnson fought uphill the whole time.
I haven't been too impressed with him on film. I think that his historical standing greatly outweighs his in-ring ability.