I think the most important quality is that they beat the best in their era.

I definitely agree that extra weight is given to a fighter who comes back from defeat.

Take Sugar Ray Robinson, 5 times middle weight world champ. He is more highly regarded than Carlos Monzon who reigned for as long as Robinson but never lost a defense. It's ironic that if he had lost and come back, like Ali or Robinson he would have been more highly regarded.