Some people might not like this, but I'm gonna say Prince Naseem Hamed. He got exploited and ran outta the sport by MAB.
you shouldn't get shot down, because it is a fair point, he put his ego and nepotism in front of his relationship with Brendan Ingle and paid the price
Older ones would include Jeffries - Johnson, Louis - Schmeling I&II (first for the 'unstoppable' Louis, second for the Germans), Liston - Ali I&II, Ali - Frazier I, Frazier - Foreman I, Foreman - Ali, also Cooney - Holmes.
Mike Tyson in Tyson-Holyfield 2
Chris Byrd in his last fight against George
David Grant in his KO lost to Lewis
Ray Leonard when Camacho starched him
Morrales in Morrales-Pacman 3
The question is what FIGHTER got the biggest reality check in recent years, not us the fans.
So hands down that has to be Gomez, he was so convinced he had a good chance, and he was so dramatically miles away from competitive with Cotto it was ridiculous, much as I like him.
2nd place would be Byrd for me. He was dead convinced in his words, he'd be doing very well a LHW and would knock top guys around, again, dramatically wrong to point of retirement.
3rd place goes to Lacy, he's not no. one for reasons stated and the above were worse personal shocks imo. But, Lacy got the longest extended exposing and shock out of the 3. He was outclassed and boxed ALL night, and you could see, and hear in his voice, he was in a state of shock.
I think this one beggers the question, if so many people thought he was so good, then there must be a few fighters out there over hyped, and I'm sorry to say, in the US. UK's been getting grief of over hyping recently so fairs fair. Many boxing people and fans believed he'd win and they were way wrong, no excuses. More than reasonable to say there must be flaws there to the point others are also over hyped.
And Hatton as a counter argument or another, well, tbh, I really don't think you'll easily find another eg., whereby most within the sport in the UK, and it's fans, genuinely thought by such a margin, they're fighter would win, and win well. You might see some hype on forums but that's talk and not the majority opinion.
How good they are not? I won't go there. But reality checks themselves have gone to Jeff Lacy, Ricky Hatton, Humberto Soto, Jorge Arce, Vic Darchinyan, Junior Witter, Zab Judah, Bernard Hopkins, Jermain Taylor, and Shane Mosley, among many, many others.