Audley Harrison. I watched him win Olympic gold back in day and turn pro. He was Ring Magazine prospect of the year. I thought he was gonna be the next Lennox Lewis. Maybe he turned pro too late, maybe he didn't have the psychological or physical tools for the pro game. But he never really got close to world titles or being a legit contender.
My personal one was Shannon Briggs. Granted he did win a title, but as he was coming through as a younger HW he was smashing people, super fast, powerful and looked like a champ in waiting. This was before all the asthma stuff and losing to the likes of Darnell Wilson(I think it was) and generally looking gassed whenever he went past 5 rounds.
Hank Lundy: Hank Lundy was a true Philly fighter, tough and skillful. I really legit thought Hank had the ability to be a world champion. Him being a Philly fighter kinda was his undoing. He was placed in so tough and had no fear of any fighter/or the outcome of the fight.
Andre Dirrell. Looked like he had the goods but just wasn't mentally strong when the going got tough. Thought he beat Carl Froch though.
As a Brit. Haye and Brook. Both had good careers but lack of activity meant they never quite got where I felt they could have. Brook chased Khan during his prime, and was questionable outside of the ring. Haye just was not active enough. He also got more negative when he was concerned with what was coming back. But had explosive power and could have been very very good. In my opinion.