In terms of reputation, Marbury, Iverson, Steve Francis all come to mind.
But for me it's Tracy McGrady. He was a dark horse MVP, and for me hands down the most dangerous offensive player in the game for a good 3 years, including Kobe Bryant. Just straight MURDER from the triple threat position, could beat you off the dribble, could shoot over just about everyone, and was just such an underrated passer who people dismissed as a playmaker because aside from Yao, he had nobody to pass to.
But he was always a bit of a diva, and at the highest moment of his diva ever in his career, his body also betrayed him. Never had Kobe or KG's durability or their killer instinct, and was cursed with a series of brittle second fiddles and terrible teammates.