Man, I hate to add fuel to this fire, but I have to support my Bears a bit.
Walter Payton - in my opinion the best all around back ever, blocking, running, passing and catching the ball out of the backfield combined.
Barry Sanders - most fantastic moves for a running back and easily could have held many records had he not retired early - but according to him, records were not important. (Gale Sayers is very close to Sanders in this "category")
Jim Brown - best power back ever, also have to take into account when he played there was more of a focus on running the ball than passing it as in today's game.
It is incredibly difficult to say who is the best running back simply because of the intangibles created by the position itself, along with the team the individuals play on. I have stated this on other threads in other sites, if someone were to create a computer program to throw in the attributes of the players with their respective teams and see how the game would be played that would be amazing.
There are plenty of RB's that could also make a top ten list for me, kudos to Emmitt for having a great career, but Walter, Jim and Barry get my nod at the position.