In their prime I might have to choose Earl Campbell, or Jim Brown. It is not a big class that contains the likes of Emmitt Smith, no one saw the field like Smith and used that to their advantage. Others were bigger and faster but not better RB's IMO.
Jim Brown was dominant in his era; however, I look at it as which one would be the best to start a team in any era. I personally don't think Jim Brown would match up to Emmitt, Barry, etc. if the all played in the same era. Imagine Emmitt/Barry playing against DLinemen that 250 pounds. Barry Church would be the fastest DB on most teams back then instead of the slowest now but he would have been a LB due to his size.
Earl Campbell would be a punishing RB in any era but his career was 8 years then. Now that defenders are even bigger his career would likely be even shorter. One of Campbell's OGs, the great Mike Munchak was 6-3, 263. The smallest starting OL today that I'm aware of is Rodney Hudson at 6-2, 300. The Nineties Cowboys were the first to have primarily 300+ pound OLinemen. Now almost if not all OLinemen are over 300.
If I'm starting a team today, who would I pick?
I've already discussed Emmitt, Barry, Earl Campbell and Jim Brown.
Eric Dickerson: He was great but defenses are faster now. He best attribute was just outrunning defenders either to the corner or downfield.
Walter Payton: Like Emmitt he could probably function well in any era because he didn't really rely on a specific physical attribute like power (Brown, Campbell) or speed (Barry, Dickerson).
Bo Jackson: He never played more than 11 games in a season due to baseball and only lasted 4 seasons due the hip injury. Having said that, he would still be bigger and definitely faster than most current NFL RBs. Imagine is Zeke was as fast as Deion Sanders (maybe faster).
I still tend to like Emmitt because he was such a complete package (rushing, blocking, receiving, short yardage, longevity).
If I need 1 yard: Earl Campbell
If had a bad OLine: Barry Sanders
If wanted amazing long runs: Bo Jackson
Who are the top candidates of current RBs to eventually make the all time great list?
Zeke: Like Emmitt he is the complete package. He style is not similar to Emmitt, but he is a very complete player.
Bell: He's been doing it for a few years now. He has missed some games and has never carried over 300 times in a season. His TD numbers are nothing special.
Gurley: He is incredibly talented, IMO, but stuck on a team that's not that good. How would he look behind the Cowboys OL?