Under Appreciated Jazz Piano Players

Cyrus Chestnut
Ellis Marsalis
Horace Silver (appreciated more as composer)
Bobby Timmons
Randy Weston
Hampton Hawes
Muhal Richard Abrahms
Ray Bryant
James Williams
Mulgrew Miller
Benny Green
Gerri Allen
Junior Mance
I reckon I could go on. Check out Ben Sidran's "Piano Players"!

I guess, by definition, "underappreciated" is someone who is less well known and shouldn't be. As nobody has mentioned Cedar Walton, I think he should be added to the list.

It seems like I'm the only one who's ever heard of her. Very Monk inspired, but more lyrical. Check her out if you haven't.

Jacky Terrasson. His collaboration with Cassandra Wilson (Rendezvous) is very nice, as are his solo efforts.

UNDER appreciated...right?
Dorothy Donegan - One of the few contemporaries of Tatum that had comparable chops...really! She was also a fantastic entertainer with an extraordinary ability to mimic jazz vocalists of various eras. Check out any of her concert recordings and make sure your mouth is taped shut.
Phineas Newborn - Nobody, NOBODY has ever eclipsed Mr. Newborn's pianism. Speed, articulation, phrasing, swing are all non pareil. Criminally underappreciated.
BTW: Thelonious labored in relative obscurity until he was 40 years old because listeners weren't prepared for him. His musical contemporaries knew he was their Mozart...or Beethoven, but it took Jazz fans a long time to catch up. We're still catching up!