Really depends on the sport. I noticed you two mentioned Lance Armstrong and Muhammed Ali...both boxing and cycling require strength in completely different areas of the body. Cardio vascular vs brute strenth...upper body vs lower body. Each sport is uniquely different that way. So it really depends on that.
But for me, I'd have to say Cal Ripkin. Most consecutive (baseball) games ever played without break, injury, or time off....talk about endurance.
-VGsatomi