Chopin Impromptu #4 (Fantasie Impromptu) is by far the hardest
one. That song is related to Beethoven Moonlight Sonata 3rd
movement. Other than Fantasie Impromptu, the most difficult is
Chopin Impromptu #3. #1 is the easiest. Impromptu #2 is more
beautiful than #1/#3. Fantasie Impromptu is also by far the most
beautiful one. Impromptu #3 is slightly more beautiful than
Impromptu #1. No offense to Impromptu #1.