I was running an errand tonight and had Brian Kenney's ESPN radio show on. He said he was talking to Mike Ditka about Favre and eventually the conversation got around to the biggest star in NFL history. This isn't necessarily the best player but the biggest star. It's easy to say that a current player would have that distinction in this 24/7 media and most of us not being able to remember stars back in the day. For my money, Joe Namath was the biggest star.
But you might have to adjust for eras. For example, Red Grange was a tremendous star when he first entered the NFL in 1925. He helped legitimize the NFL when he joined the Bears and they went on a 19 game 67 day barnstorming tour. He got in a salary dispute with the Bears and left to form his own league. How many players had the influence to form their own league? Someone like him would be the biggest star of the 20s. Maybe someone like Bronko Nagurski would have been the biggest star of the 30s. Maybe someone like Don Hutson or Sammy Baugh for the 40s.