I think Emmitt Smith is by far the most overrated runningback in the history of the NFL. He ran behind the best line of probably all time. All he had to do was pick a hole and run through it. I think he was exposed in his later years with Dallas and the Cardinals. Sure people say he is old, but I think it is more then that.
I am not a big fan of Jim Brown either. The guy was simply way ahead of his time as far as athletism, which never happens today. Barry Sanders and Payton are by far the best two backs of all time.