Roy Jones Jr and Floyd Mayweather Jr for me. Jones had Robinson like abilities, but spent the early part of his career under his Father's naturally cautious guidance. Then, post Toney, when he could of beaten anyone under 200lbs, anywhere, he avoided the matches that would of made him a top 10 great, rather than a top 50 version. Mayweather too, after an excellent start, became diva-ish with whom he was going to fight. He also jumped up divisions too quick, and thus did not establish proven dominance in divisions.
Jack Dempsey maybe? The millions and millions of boxing writers from when men were men ALL claim he was Achilles reborn. Yet he went on to duck an entire ace of people, put his title to sleep for 3 years, got KOed by a fireman/part time boxer with a losing record and was schoole by an obese ex flyweight. Didnt meet the top 2 contenders of his entire reign either
James Toney and Mike Tyson, as great as they were, could have achieved so much more with their incredible talent. Also like to add: Donald Curry Michael Nunn Gerald McClellan and most recently Andre Dirrell