Walter Payton...
The guy ran behind an absolutely atrocious line for the first 5 years of his career. He WAS the Bear's offense for many years. No running back could catch a ball, absolutely truck defenders to gain 5 yards and then turn around and throw a perfect spiral to the end zone the very next play like he could. Payton missed 1 game his entire career. How many RB's today could take the wear and tear he did( especially since he sometimes had to run 300 yards a game to actually gain 175) and be half as productive as him? None...that's how many.
Now if you are talking pure running backs then yes, Barry Sanders is the man. I will always respect Barry Sanders...he retired early so that he wouldn't break Walter's record...that's how much respect Sanders had for Payton's talent. But if you are talking about the best athlete that ever played the RB position? Payton is in a class by himself.