There are many songs that are considered difficult to play on the piano. Here are some of the most challenging pieces according to various sources:

1. Piano Concerto No. 3 by Sergei Rachmaninoff
2. "La Campanella" by Franz Liszt
3. "Take the A Train" as performed by Oscar Peterson
4. "Opus Clavicembalisticum" by K. S. Sorabji
5. Beethovens Sonata op. 106
6. Alkan etude Op. 39 No. 1, Alkan etude Op. 17
7. Scherzo no. 1 by Chopin
8. Ballade No. 4 by Chopin
9. Prokofiev concerto no. 3
10. Barber sonata
11. Stravinskys Trois mouvements de Petrouchka or Three Movements from Petrushka
12. Messiaens Vingt Regards sur lEnfant-Jésus
13. Liszts Transcendental Etude No. 4

Its worth noting that the definition of "hard" varies from musician to musician, and some of these pieces may be more difficult for some pianists than others. Additionally, some of these pieces may be inaccessible to most people, as they require years of training and practice to master.