For me it has to be the gifted one Kirkland Laing.The man was a sublime boxer but unfortunately did not have the dedication or discipline required to fulfil his great potential
To many too name. But here's a few. Clint Jackson Tony Ayala Ike Ibeaubuchi to name a few, prison cut short their careers.
There was a middleweight who was a four time gold glove champ and fought on the U.S. boxing team on around 79 and 80 Alex Ramos, he seemed to have some good tools but I didn't think he was dedicated enough and lost to some people he shouldn't have
Honestly, I would say Roberto Duran in a sense. Hear me out, he didn't completely underachieve but I don't he has reached his highest potential. His resume is already enough to make it to the Top 10 Pound for Pound Fighters of all time. I'm pretty sure y'all are very familiar with his resume: Legendary lightweight champion reign (beats Hall of Famer Buchanan and the tough Esteban de Jesus, 12 title defenses mostly by stoppage-->cements Duran as Top 3 Lightweight of all time), moves up to welterweight and had success (beats out of prime but still game Palomino and Cuevas, BUT a PRIME Sugar Ray Leonard--> Cements Duran to Top 20 Welterweights of all time), and had limited success going beyond welterweight (Mauling Davey Moore and outclassing Iran Barkley). Some critics will pick apart Duran for "No Mas" and losing to everyone in the Fabulous Four. It's understandable that a naturally smaller man gets beaten by the naturally bigger man. Imagine if Duran beats everyone in the Fab 4: Knocks out Leonard in the rematch, becomes the first man to knock out Hagler, and then knocks out Hearns instead in the 2nd Round. And imagine if Duran didn't have any upset defeats to fighters like Kirkland Laing or Robbie Sims. Also, let's add that Duran knocks out Wilfred Benitez at junior middleweight and that Duran has 12 Title Defences at Middleweight, tying the amount of title defences he made at lightweight. I feel that if Duran was able to do all that, he might even rival Sugar Ray Robinson, Harry Greb, and Sam Langford as the boxing GOAT. I feel given the guys that Duran fought in his career that Duran had the potential to become a Top 3 Pound for Pound Fighter of all Times or maybe #1. But because of his lack of discipline later in his career and mercurial nature beyond lightweight, I can't rank him among the Top 5 P4P but he still remains within the Top 10 P4P of all time. Disclaimer, I am not trying to diminish the legacy of the great Roberto Duran. He is and always will be one of the best to ever do the sport but I feel he could have done a little more with his career.
Clint was a few years out of boxing (as a combatant) when he was convicted of kidnapping (an event which occurred, also, some considerable time after he had stopped fighting). And his career had fallen into decline starting with his 1983 KO loss to Buster Drayton. He fought on until 1985, losing 5 of his last 7. The kidnapping was in 1988. Clint was coaching amateur boxers in Nashville after retiring from the ring. Maybe some pros too, not sure.
Perhaps he could have popped up to 175 and KO'd Spinks as well. Don't think he'd KO Holmes tho so he'd have to settle for a comfortable UD.
I do not get the Ibeabuchi call, it is a lazy generalisation. He was mentally ill. That helps explain some of the tactics he used. Without his illness, and thus potentially a later retirement, he would have been a slow plodder with a decent punch and good chin. That probably gets him a top 10 ranking somewhere along the line, and with the right promotion, a losing title shot. Thus Mr President overachieved greatly.