David Tua for me has to be up there. The guy was a case of the right guy for the wrong time. He beat multiple former or future heavyweight champions in Ruiz, Rahman, Maskaev and Moorer. Excluding the tail-end of his career, he had one of the highest KO percentages in the history of the HW division with a left hook that would make Joe Frazier proud.
When the time finally came for him to get a title shot, he was unfortunate to have to fight Lennox Lewis for the title. There were much lesser champions in the 90's/early 00's that Tua could have easily beaten to become world champion, but Lewis was a master of his craft and beating him was a tough ask for anyone. Ultimately, he never won a title but had his opponents come at a different time in his career, he could have been a multiple time world champ.
Ike Ibeabuchi is a very big what if in boxing. Ended his career with a perfect 20-0 record before being arrested. He beat David Tua in a close back and forth war of attrition. The final fight of his career - he beat Chris Byrd from pillar to post, but went to prison shortly after and we never really got to see him against some of the other big players of the time like Tyson, Lewis or Holyfield. I've no doubt that if he stayed out of trouble he could have very well been world champ.
Jerry Quarry is another case of the right guy in the wrong time. Jerry fought in one of the the strongest heavyweight divisions in history with the likes of Ali, Frazier and Foreman ruling the roost. The guy had relentless pressure with an iron chin. He has wins over Ron Lyle and Earnie Shavers, two of the hardest hitters in boxing along with a win over former champ Floyd Patterson. He was a little small for the division, regularly fighting at under 200lbs and cut easily. Probably his only downfall.
Ken Norton - This one might be a confusing choice for most boxing fans. Ken Norton never won a world title. It sounds strange to hear it, but it's true. He held a world title, but never won it. He was awarded a title after Leon Spinks was stripped of WBC belt, having won an eliminator fight with Jimmy Young. But he never beat a champion for one. I can't think of anyone who gave Muhammad Ali more trouble over his career (with perhaps the exception of Joe Frazier) than Ken Norton. He arguably beat Muhammad Ali and gave a prime Larry Holmes hell in their fight losing a split decision that nobody would have argued if it went Norton's way. It could be said he was often fighting the judges more than his opponent.
Who else do you think should be on the list?