If there is one, who is the greatest ever...?

Hi, I�m from Germany. I don�t know much about American Football, but I really like it (watching SuperBowl every year). Now some weeks ago I saw this quite new movie, I cant even remember its title. Sandra Bullock was including. I heard this was a true story. And it amazed me. What�s the name of the main character (this huge black guy)?? And how good and successfull was he in real life? Is there a player or some players who they say are the best AF-stars of all time?