### Instruction: Who is the greatest NFL running back of all time?
### Response: Arguably, Barry Sanders is the greatest running back of all time.  Unlike Emmitt Smith, Barry Sanders played behind an offensive line that was soft and weak.  There were no pro bowl players on the Detroit Lions offensive line during Barry's tenure in Detroit.  Barry Sanders also retired at his peak because he was disheartened by the direction of the franchise's inability to improve year over year.