Zab Judah is the very first person that pops up to mind . With his skill level, He had no business losing to someone mediocre like a Baldomir .
He's still an active fighter but I'm confident saying that Kirkland could have achieved more when it is all said and done.
Gerald McClellan. He was a great boxer. Freaking Nigel Benn crippled him with his steroids. It's unbelievable he's walking freely.
The fighters that jump out at me are Bowe, (in shape he could have had a much better career, especially if he had fought and beat Lewis) Tyson, (could possibly have been close to the best heavyweight ever, if he kept his training up) Benitez, (won his first title at 17 and was washed up in his mid 20's after winning titles in 3 weight divisions. Ike Ibeabuchi (jail's not a good career move) and Duran. If Duran had great training habits from 30 years old onward he could have been in the discussion as to whether or not he was the best ever. He was that good. Toney is another great shout out. It's actually amazing what he did considering his weight issues. That fight against Jirov was one of the best fights I ever saw. It was an amazing battle and it should get more recognition than it does.
Chavez jr (discipline) Penalosa (management) Judah (i dont know what he lack but got a feeling something is missing with him) Espinosa (management) Valero (discipline) Thats the guys in top of my head atm
Judah was the first that jumped to mind. He had ATG speed, reflexes but the mental fortitude of a midget. Guzman Ortiz Holt Chambers