Jack Johnson was among the very best.
Who did Marciano fight that was top quality in his prime? Ezzard Charles comes to mind.

It's ironic that Jack Johnson was denied passage on the Titanic because of his race. Apparently his love of white women saved his life.

Ok, how do we define the greatest?

I think Mike Tyson could have been the greatest of all, but as I said at the time, only Mike Tyson can defeat Mike Tyson.