I don't know if there is a definitive "greatest" accomplishment but I think Otto Graham's 10 Championship appearances in the 10 years he played (Cleveland Browns) is pretty damn good. Won 7 of those games.
"Automatic Otto" never missed a game as a pro while passing for 23,584 yards and 174 touchdowns. Graham finished his career with an astounding 105-17-4 regular-season record.
http://espn.go.com/classic/obit/s/20...7/1688946.html
That is pretty much the epitome of greatness right there, in my opinion.