I like Opp. 2/39/76/116 the most for Brahms.
But randomly I would not consider any piece the "greatest piece". How can you compare the "greatness" of Bach Goldberg Variations to Debussy Estampes? My
favorite piano pieces are:
Alexander Mosolov- Sonata No. 2
Iannis Xenakis- "Herma"- Musique Symbolique
Tristan Murail- Territoires de l'Oubli
Karlheinz Stockhausen- Klavierstuck XIII
Arthur Kampela- Nosturnos
Alan Yim- :[ten]dril
Nikos Skalkottas- 32 Piano Pieces
Alexander Scriabin- Sonata No. 6
Alberto Ginastera- Piano Concerto No. 2
Gyorgy Kurtag- Jatekok
Pascal Dusapin- Seven Etudes pour Piano
Michael Finnissy- Folklore
Michael Finnissy- Solo Piano Concerto No. 6
Toshiro Mayuzumi- Pieces for Prepared Piano and Strings
Sylvano Bussotti- Due Soliloqui

Hans-Joachim Hespos- -Z...()
Luigi Nono- ...sofferte onde serene...
Salvatore Sciarrino- Nocturnes
Yoshihisa Taira- Sonomorphie I
Andre Boucourechliev- Anarchipel 5
Luigi Verdi- Frammenti
Kaikhosru Shapurji Sorabji- Prelude, Interlude and Fugue
Favorite Standard Pieces:
Wagner-Liszt Tannhauser Overture
Ravel Gaspard de la Nuit
Prokofiev Toccata Op. 11
Debussy Images II
Schubert Sonata in A Minor D784
Liszt Ballade No. 2
Chopin Piano Concerto No. 1
Bartok Sonata for Two Pianos and Percussion
Meyerbeer-Liszt-Busoni Fantasy et Fugue
Alkan Trois Grandes Etudes Op. 76
Alkan Symphonie pour Piano Suel Op. 39
Prokofiev Sonata No. 4
Beethoven Sonata in E Op. 109
Franck Piano Quintet
Scarlatti Various Sonatas (K. 417, 450)
Soler Various Sonatas (No. 49)