Based on the list, Howard Davis jr.That joker should have been able to win at least one world title despite his deficiencies.
I cant help but think that if this question is asked in a years time Adrian Broner would probably find a berth prospect Francisco Bojado seemed to disappoint a lot of fans Likewise Ed Hopson could not make the Transition In the UK Errol Christie seemed to have it all...until his chin was found wanting
From a "boxing as a business" standpoint it would probably have to be Jermain Taylor. HBO was all-in on that guy when he dethroned the long-reigning Hopkins and they were expecting huge things out of this big, rangy, powerful middleweight. He seemed to have all the tools and should have been poised to reign for years, giving them another reliable "house fighter" they could shill for a while. You don't beat a guy like Hopkins by accident, after all. He just never developed.
Tommy Morrison, he beautifully out boxed Foreman, then KO'D Ruddock, then in 1996 all he has to do is beat some journeyman, (which would have been a walk in the park) and he lands the biggest heavyweight fight in history against Tyson, worth zillions BUT he caught the AIDS virus, and boom everything goes down the shitter.