Randy Moss. The most dangerous offensive weapon in football history. We'll never know his full potential because no QB has ever been able to take full advantage of it. Brady is doing better than everyone else has and showed us a better glimpse at the real possibilities, but even Brady often underthrows him when he's behind the defense. The guy who coached him when he trained for 2 days to win the southern conferece indoor track meet was Carl Lewis's olympic track coach as well, and said that Randy is the best runner he's ever seen. He won the state's Mr. Basketball title twice in highschool while playing with a guy who would go on to be in the NBA. And they say that baseball was actually his best sport. I've seen video of him playing around with a soccer ball and scoring goals on people and he's good at that to. God only knows what other sports he would've been pro quality at. There are several guys with ridiculous amounts of athletic ability, but I can't think of many who could translate it so well to different sports.
Another guy who always comes to mind is Mike Tyson. We never saw what his prime would've been like. We never saw anything close to a prime with him. And yet what we did see was probably still unbeatable at it's best. Really sad to think about what was wasted there.