MMA I would have to go with Kimbo. They (elite xc) based their entire company around this fellow. He gets KO'd in 14 seconds by a guy a weightclass below his, and the company folds

As useless as Bob Sapp was, he does have 2 wins over K-1 legend Ernesto Hoost and a legendary MMA loss against PRIME Nogueira, so even though he was marketed and promoted to high heaven, he did have some good performances along the way.

Gerry Cooney suffers from that "white hope" double sword. When he was winning they overrated him, and when he loses they underrate him. Gerry was a REALLY good fighter IMO, dynamite left, good jab. One of the most brutal KOs of all time against legend Ken Norton. Look at the 3 guys he lost to. Holmes and Foreman, who IMO are both top 5 HWs of all time, and Michael Spinks, one of if not the greatest LHW of all time, and a guy who moved up to HW and handed Holmes his first 2 defeats. Yeah he looked bad but it was against STELLAR compedition.