Ali or Tyson. Afterwards... Marciano. ........ Everyone on the planet knew who they were.. Superman vs Muhammed Ali.. and so on.. Only probably 2% on the planet now knows if Mayweahter is a runner, boxer or basketball player..Even Oscar was mainly known in the latino world. The rest of the world outside of boxing would have no clue.
Leonard opended the doors for non-heavyweights to take home multi-million dollar purses, indeed. He was a megastar.
Ali is the clear cut winner, but don't sleep on Joe Louis. His fight with Schmelling, at the peak of the radio age, was one of the first globally broadcast sporting events. He was to radio what Ali was to the tv age.