My personal work for piano if i had to choose ONLY ONE it is the solo piano transcription of ravel Rapsodie espagnole made by Kaikhosru Shapurji Sorabji (yes the man who wrote Opus clavicembalisticum) (i love the second version of that work rapsodie espagnole composed at 1945).The reason is that we all hear all the piano works of ravel,played frequently by any pianists.And it is kind of boring some time to hear the same and same works.It is not the works that are boring but rather the level of the people who play,who may have technical mastery,but the music is gone out of the window.Now as i know ravel did a 2 piano version of that work,he never composed it for solo piano.
Sorabji's transcription is much better (!!) than the 4 hand version of ravel.He managed to recreate the whole orchestra atmosfaire,in to a very difficult piano transcription (who has 3 staves all the time,knightmare to play it believe me especially the last movement the feria!).
Even in places where it is impossible to transcribe for a single piano,sorabji (i dont know how!) managed to do so,it sounds like 2 people are playing at times (listen to malaguena,or the habanera).Feria is all the money my friends,and it is a 100% transcription of that work,it is not a paraphrase or a mix of sorabji's own ideas (like a fantasy on themes or something).
There are other works that i love and would love to play or study but this one stole my heart.