Joan Guzman is a great call if you limit it to just wasted talent. He was an incredible fighter for a time and could have been very live in fights with Pacquiao or Marquez, but never did much of anything. Obviously he was a lazy idiot and screwed his own chances, and there are loads of talented guys who just don't dedicate themselves like that.

On the absolute other end of things, how about Mike McCallum? Obviously had an incredible career and beat all kinds of killers, but he was imo one of the greatest fighters ever and was unfortunate to never land a few huge name fights at his peak. He could have fought SRL, Hearns or Duran and beaten them imo, could have got a fight with Hagler etc. Was very unlucky to lose his 0 in the first fight to Kambalay, and was a bit older by the time he fought Toney although still won the first meeting there imo. Guess he didn't quite have the star potential of a Leonard, Hearns or Duran but he was every bit as deadly and is criminally unheralded.