When I see the phrase "pound for pound" it makes me think of the most complete player at the running back position. That complete player was Walter Payton. And he did it all with little around him on offense, and never wanted to go out of bounds on a running play before he smacked you in the mouth first.
While a whole list a running backs can be considered the best of all-time, I look at everything a player can do on the field. What most people forget is that Walter Payton was a place kicker, punter, and often threw passes in the Bears offense or while attending Jackson State. He also led the league in kickoff return yardage in his rookie year. If he played for anyone else that was a perennial Super Bowl contender when he came into the league, he wouldn't have been asked to do so much for the Bears.
Not one single Bears player on the offensive side of the ball for Payton's first ten seasons in the league made the Pro Bowl. And more specifically, not a single skill position player made it either during his career. Someone tell me who else could carry an offense like this for the most productive years of one's career with so little around him?
It wasn't until Mike Ditka came along that the Bears finally surround him with a decent quarterback and offensive line. And more importantly, they finally had balance. All this while Payton had carried the ball more times than anyone else at that point when they finally got him some help. And let us not forget that he only missed one game in his entire career 13-year career which he could have played in anyway.
It's always going to be argumentative as far as how you determine the best running back ever. Sanders would definitely be the most elusive. Jim Brown dominated the field when he played. There are many that can claim they were the most powerful runners. But as far as a complete running back goes, I would go with Payton.
Payton was the Bears offense, and he still could not be stopped. Not only that, he could beat you in so many ways with running, receiving, passing, kicking, or blocking with not having a good cast around him, too. It is for that reason while I think he was pound for pound the best running back.