For me, it's Joe Namath. It seems he's only in the Hall of Fame because of one game. He pulled off one of the biggest upsets in NFL history in beating the Colts in Super Bowl III and also brashly guaranteed victory beforehand. That alone made him a legend.

However, the rest of his career was very mediocre. He threw more interceptions than touchdowns, had a passer rating of 65.5, completed only half of his passes, and also had a losing record as a starter.
As a matter of fact, I think he finished with a winning record only twice, one of those years was when he won a Super Bowl. His overall track record was not that impressive.