OK, I'll bite. Who?
There's nothing he can't do as a running back. I'm not sure I've ever said that about any back. I didn't see Jim Brown play live, but I did notice in the highlight reels that he was running against a bunch of dudes that looked like my dad.
Barry didn't have his power. Dickerson didn't have his quickness. Payton didn't have his size. Emmitt didn't have his speed. OJ didn't have his vision. Bo didn't have his ability to catch. Marshall didn't have his ability to pass protect.
Peterson literally has no flaws. He is the perfect running back.
He's not better, IMO, then Jim Brown. You can say that the talent level was inferior but that also carries over to the quality of OL. Add to that, the defenses Brown ran against were geared towards stopping the run so you really didn't have schemes that were more concerned about defending the pass. You can make the argument that it was harder to run in those days.
Walter Payton was not as big but was just as powerful. He could do it all and he did. Everybody remembers Walter Payton on those great Bears teams of the Mid 80s but Payton also played on some of the worst Offensive teams I've seen fielded in the NFL. He was it and he was awesome. I am not sure there has ever been a better all around back then Payton was.
Dickerson was faster IMO. When he hit a crease, he was gone. A different kind of back but production is actually very similar if you look at the numbers in the first 8 seasons of each player.
Sanders didn't have his power but AD doesn't have Sanders' elusiveness either. Barry Sanders put up better numbers in his first 8 seasons as well. Interestingly enough, in his 9th season, he ran for 2053 (44 yards less then ADs best season) with a 6.1 AYPC. Sanders was still going strong when he decided to retire (like Jim Brown), and could probably have continued to still be highly productive. We'll never know how much longer.
Emmitt didn't have his speed but he didn't have Emmitt's balance. Time will tell if AD has Emmitt's durability and Emmitt has the record.
OJ had better vision. There are only a hand full of backs that I can put in the same class, as far as vision goes, with OJ. AD is not one of them IMO.
Bo is not in the discussion IMO because he didn't play long enough and, he never fully committed to Football but if he had, I don't think anybody would have been better then Bo. Bo could do it all, including catch the ball. He only started 23 games his entire career. Keep in mind, Bo never came to training camp because he played Baseball. He came in, during the season for the Raiders. He never really had a lot of work in the passing game so was not incorporated into it but if you look at his College career, he caught 272 passes for a total of 4575 yards. Bo could catch the ball.
Marshall was pretty good as well. He was in a passing offense so that is a disadvantage for him but the other side of that is that AD could not do the things that Marshall could do in the passing game.
There are others LaDanian Tomlinson had more production in his first 8 years. There are others.
If you are saying that there is nobody like AD ever, that's OK but that doesn't necessarily mean he is the best ever. There has never been anybody exactly like any of the players mentioned here. AD is a great back, no question but he's not the greatest I've ever seen at this point in his career IMO.