Johnson.

Dempsey had nothing to hold him back fighting his best in that era and whay we saw of dempsey was all he could be.

Jack johnson was carrying fighters and was 60% of the fighter he was for fear of being killed at any point due to his black skin.

They both have different respectable styles but you have to factor in the outside influences and the fact jack destroyed everyone in his prime with ease but was holding back so much more.

Such a shame.