How are Holyfield, Hopkins, Marciano, Lamotta, Frazier, or anyone else for that matter special?

Special fighters are the guys who have enough talent, and immense dedication and determination for the sport.

Bradley is special because he works as hard as anyone in the gym, and he makes as much with what he has as he can.

There is always somebody who faster, stronger, etc. That isn't why guys like Mayweather, Martinez, Pacquiao and Donaire are at the top. They are there because they can do everything else better than guys who are specifically better at one thing. I used to look for one quality that defines the greatness of a fighter, and there isn't one. It's a mixture of things coming together for a determined individual that makes a great fighter.