Sayers simply did have the years (number of) that should be there to consider him one of the greatest all-time. Before he blew out his knee, he was fantastic though. Sanders was as explosive, and he was durable.
Payton, Smith, Dickerson, Simpson, etc, were all great players, but the absolute best RB of all time is still the great Jim Brown. No one has ever been so thoroughly dominating as he was his entire career.
Emmit was a shell of himself the last 3 years when he was chasing down the All-time yardage record. He also had the privilege of playing behind a huge offensive line in his heyday. Had Walter Payton played in Dallas during those same years as did Smith, I firmly believe his numbers would have exceeded Smith's considerably.
#1 - Jim Brown #2 - Walter Payton #3 - Barry Sanders