Vin Baker, for sure.
Also, I think Chris Weber qualifies. He was regarded as "the next big thing" when he was drafted. It is true that he had some injuries throught his career -- even early --, but the fact is that he never really developed his game. For instance, despite being a natural ballhandler, his post moves remained raw and he remained an inconsistent jump shooter... As a result, even in his best years, he was not an efficient scorer. For someone hyped as he was as a prospect, I think he underachieved.