Instructions
Open the Creation Zone in Madden and select "Create New Player." Create a running back and set the year to: "Rookie." Set him to be on your franchise team.
Max out all of the stats at 99, the highest they can be set, and save your player.
Exit the Creation Zone and open "Superstar" mode for your team. Select "Use Rookie as Superstar" from the options. Find the running back you created and select him. You now have the best running back in the game.