The Best Piano Works of the 20th Century

Ravel's - Gaspard de la nuit and Debussy' Preludes. Both are post 1900 as are most of these composers music for solo piano. Their music should not be missed. 

I love "Gaspard" and the Debussy Preludes.  I think Satie is also worth looking into, although his most popular works were composed in the late 19th century.

Gershwin's "Rhapsody in Blue", a uniquely American piece always has to be included in 20th century piano music. I agree with all of gregm's choices less  the Massiaen piece but will sure check it out! The impressionistic work Clair de lune by Debussy  would be on my list as well, less clear is when it was penned.

I was looking for pieces for solo piano, but I wasn’t clear. Regardless, there are some great suggestions here. Nancarrow is an excellent suggestion. I’ll add Prokofiev’s Piano Sonatas. 

I overlooked Prokofiev’s. :-(  FWIW I highly prize Chiu’s 9 CD traversal of all of his music for solo piano including some transcriptions. I also overlooked Rachmaninoff’s 24 Preludes and Sonata #2.