Floyd Mayweather Jr. (WSJ article)

The Wall street Journal names Floyd as the #3 most fit athlete the categories the pro athletes were scored on were:
Vision and Reflex
Stamina and Recovery
Power, Strength and size
Speed
Success and competitiveness of sport as a whole
Coordination and Flexibility.

Obviously the WSJ is not on the front lines of sports or science, but it is an interesting way the were trying to get like a p4p rating on all athletes.

WSJ.com

The only other boxer I saw listed was Wlad Klitchsko who was in the power, strength and size category.