Ryan Leaf
San Diego thought that they pulled off a coup when they drafted Leaf in 1998. The former Washington State prospect had exciting raw potential. Peyton Manning was the big name that year but many thought that Leaf also had an amazing long-term ceiling (via Sportskeeda).
But his horrible performances derailed the hype train and they threw him away like yesterday’s garbage. Everybody knows about Leaf’s addiction problems but maturity was also a big issue. He spent a couple of seasons bouncing around the NFL as a backup before retirement.