Jim Brown was the most dominant in his era. I'm really not sure how anyone can argue otherwise. And I think the only fair assessment of players, era to era, must include normalization.
Things were very different when players had run of the mill part-time jobs in the off-season. Read Tim Cowlishaw reminiscing how he and Drew Pearson worked together in a Northpark Mall sporting goods store, while Harvey Martin worked at a shoe store.
Add to that normal human evolution, diet, training. etc. We must assume that a Jim Brown born 20-30 years later would have benefited.
Long before there was an Internet, I recall Tom Landry being asked what contemporary RB would best fit his system. I believe he named Jim Taylor, the great Packers back. But he said: "If I had Jim Brown, I'd change my system."
I would take Emmitt over Barry Sanders. The object of a football game is to win, and players should be judged on their contribution to that goal. Emmitt was the kind of back with which you win. He hit enough home runs to make a defense account for that ability. He made the tough yards. He minimized negative plays. He blocked. He was durable and had longevity.
Barry Sanders was more apt to hit the home run. I don't think he did anything else as well as Emmitt did.