蛇吐 蛙 by 原口 has a great design and concept.
And then 日ノ本卍陛 by 黒野エージ is one of my favourites. Simply because he's a yankii with a rough, difficult to use voice.
I'd say roughly 99% of UTAU could be considered "underrated", though. Since a vast majority will never be used by anyone aside from their creator.