RB is such a hard position to judge because your success depends a lot on others.
Did you have a good offensive line?
Did you have a good passing game to keep the defense honest? But not so good that they pass all the time?
Did you have a coach that preferred to run the ball?
Did you have a good defense, which allowed you to run more in order to kill the clock?
And more recently, do you have a coach that preferred to run with 1 guy, instead of keeping his star fresh by issuing a committee?
With all those factors involved it's almost like you just need to judge RBs on their highlights. In which case I'd say Barry Sanders was the best.