As I said before, I rank the top-3 RBs as:
1a. Emmitt Smith
1b. Walter Payton
1c. Jim Brown
But there is so little separating the three that you could rank them any way you choose and not be wrong.
The reason I give the nod to Emmitt is not because he was a Cowboy but because he has the body of work to support it. He is the all-time leading rusher in NFL history, has scored more rushing TDs than anyone, was consistently near the top longer than anyone, won more than anyone, and was outstanding in EVERY phase of what it means to be a RB in the NFL.
Payton was the best all-around athlete of the three and Brown the most dominant runner of them, but neither of them could block as well as Emmitt. Only Brown could rival Emmitt near the goalline but he didn't play as long as he could have and quit on his team just like Barry Sanders would many years later. Considering his size, Brown was not even a good blocker although he was an excellent receiver.
When I rate each player in terms of the various aspects of a RB Emmitt comes out on top, barely.
Of course that is only my opinion but I have the advantage over many of you in that I saw Jim Brown play for the last few years of his career whereas most folks here only get to see highlight clips of him.
People get too enamored with Barry Sanders' average per carry or the number of big runs he had and ignore the fact that he is the all-time leader in negative yardage by a RB and that he never won anything. The guy was a great runner but not a truly elite RB as he was taken out in goalline situations which is the main reason why his average per carry is so much better than Emmitt's.