I voted for Jim Brown.
9 NFL seasons = 9 Pro Bowls, 8 times selected 1st team All-Pro ( in '62 Brown had to settle for 2nd team All-Pro
); 3 time league MVP;
averaged 5+ yards/carry and 100+ yds/gm for his career; retired while still in his prime as the NFL's all-time leading rusher, a record he held for 21 years...all this while never missing a single start.
Brown is not merely the greatest rb, he's the greatest player in NFL history.