Brett Favre's stats speak for themselves. Based on those alone he's a Hall of Fame caliber quarterback no matter what his attitude while on the field was. However, the fact that he always appeared to be having the most fun possible, and that he seemed to be eternally grateful to be playing professional football just adds to his legacy.
Sure, he could have hung it up before the start of last season when critics were coming out of the woodwork calling for his retirement. But his competitive spirit wouldn't let him end his career after a disappointing season. Instead, he came back for one more try to see what he could do.
Many people thought that he would get hammered all season long with what looked to be a mediocre team, and that we would see Brett ride off into the sunset bruised and battered. However, something happened during the offseason, and through the first few weeks of the season. Whether his teammates wanted to send him off in proper fashion, or whether his skill just elevated their play(or both) I'm not sure. But the team exceeded what many "experts" had predicted, and gave Brett the opportunity to make the playoffs one more time and almost get into another Super Bowl. Brett's last game wasn't necessarily his best, but he showed all the naysayers that he still had "it". And his recently announced retirement lets him ride off as a great competitor that always gave 100%, and knew when it was time to get on his horse.