The Cowboys have been blessed to have TWO of the greatest RBs to ever play the game. Over their careers they averaged similar YPA (Dorsett - 4.3, Emmitt - 4.2). Dorsett seemed more potent in the passing game (YPC - 8.2, Emmitt - 6.3) while Emmitt did a considerably better job of protecting the ball (61 career fumbles to Dorsett’s 90). Dorsett was capable as a blocker but Emmitt was outstanding in pass protection.
It’s a tough choice but I have to go with Mr. Longevity here, Emmitt Smith. Maybe the greatest Cowboys player EVER - Smith is definitely in that conversation.