Gonna go with Oscar Tshiebwe. National Player of the Year -- we've only had two of those. He's probably one of the three or four greatest players that has ever played at the school, but people tend to focus on the things he wasn't great at. He wasn't an elite rim protector (but was better than given credit for) and wasn't an above the rim interior finisher, but deserves more praise than he gets. Sounds weird to call a NPOY underappreciated, but he certainly was IMO.
As far as another player I'd consider underappreciated -- Erik Daniels. He wasn't an outside shooter, but was an elite passer, could switch defensively onto any player in the country, could score in the post, finish with either hand, rebound, slash, could play SF, PF or C, and did all of the little things that make a good team a great one.