A few other great piano solo suggestions:
Elmo Hope, Hope Full
Earl Fatha Hines, The Father of Modern Piano (5 LP Box Set)
Cecil Taylor, Silent Tongues.
Elmo Hope, like Herbie Nichols, was mostly overlooked while he was alive. He died young at the age of 43. Thanks to A.B. Spellman's excellent book, Four Lives, Herbie Nichols has become better known. Like Herbie Nichols, Elmo Hope was influenced by Theloneous Monk (and Bud Powell).
Earl Hines had a long career beginning in the late 20s. He was influenced by Teddy Wilson and the great Art Tatum. He had a comeback in the 1970s, when Earl Hines was in his 70s.
Cecil Taylor's style is more avant-garde. Some years ago, I heard Cecil Taylor play in a small theatre in Santa Fe, NM. I was blown away. His virtuosity is unsurpassed.
I love Bud Powell's music, particularly the Amazing Bud Powell series on Blue Note and the Essen Jazz Festival Concert. However, I am not familiar with any of his solo piano music.
I have not heard Art Tatum's 20th Century Piano Genius. Based on Waltersalas' recommendation, I will try to find it.