Mark Westcott. I'm not exactly sure where to post my opinion on him, as he is still alive (as far as I know), but I believe is not having the same career he was once having. He is known mostly in particular circles, and within those circles, I believe his personality and personal life (to some extent) is as striking as his best playing (at least what people would generally probably consider his best playing). Now, I think he is not necessarily just "underrated" per se, as much as he is just not widely known. I think he is a pianist whose ability is to be celebrated still, and to those who happen to have known him in perhaps distant years-past now, as well as/or alternatively have access to his non-published recordings, his ability is greatly admired, respected, and even cherished, I believe (and therefore, not truly underrated).
I personally think he is a brilliant player with the soul of a young, curious and powerful wizard. I dearly hope to know him in the long line of life, and to see him find himself again, and to be free.