When the Ravens play the Minnesota Vikings on Sunday, they won't just be playing the best running back in the NFL. They will be playing one of the best in league history. Adrian Peterson is only in his seventh season, but he already has a resume that stacks up against some of his Hall-of-Fame predecessors. Has he already ascended into the top five? In this week's Five for Friday, Matt Vensel -- with a little input from colleague Aaron Wilson -- ranks his picks for the five greatest backs in NFL history.