I'd put my money on Marciano, Johnson and Ali for top three.
What about you?
Say what you will about the nutball thug, but Mike Tyson in his prime has to be ranked in there as well. Nobody could even touch him. Too bad he didn't have superior rivals to define him, as the greats you mention had.
Those would be my top three as well, although it would be difficult to pick an order. .....Holmes could also be up there.