Bobby Hurley.
Marvin Williams was drafted second overall and was expected to become an all-star.
A few other dudes that come to mind:
Borisov Diaw
Andray Blatche - he could have been great
Baron Davis
Antoine Walker - turned into a lazy chucker
Tim Thomas
Thabeet
OJ Mayo
Brandon Jennings
Dante Exum
Kanter
Jeff Green
Ryan Games
And possibly one of the biggest would be Rashad McCants.