Sonate in b minor
Chopin's music mostly illustrates the nationalism in Romantic era. He was the most prolific Polish composer. He wrote pieces exclusively to piano.
there were no pianos when JS was alive
* Halberg Suite * Lyric Pieces (piano) * Peer Gynt * Piano Concerto in A minor
Mozart's sonatas are actually among the least of his compositions. They are beautiful, but the concertos are more significant as piano music, and among his total output they are not among the most important works by any means.
He wrote piano music, solo (with piano accompaniment) pieces for violin and cello. He wrote pieces with orchestral accompaniment for piano, violin, and cello. He wrote chamber music for string quartet and for piano trio (piano, violin and cello). Most of his music is orchestral and operas.
I would probably say Fur elise by Mozart
Generally, three of Chopin's most famous pieces include his Nocturne in E flat Major, his Prelude in D flat Major (also known as the Raindrop Prelude), and his Fantasie Impromptu. As far as "important" pieces go, Chopin has written a multitude of pieces that could be considered important, as he was a cornerstone in the Romantic era of piano music. Specific ones that come to mind are his Military Polonaise, his Ballade in g minor, and his Etude in E major (this piece is especially adventurous, even for the Romantic era. It was also Chopin's personal favorite melody).
Schumann's most popular solo piano pieces--"Kinderszenen," "Carnaval," and "Faschingsschwank aus Wien"
a piano
I wouldn't say it was the most important, but probably the one piece that is given to every beginning piano student, making it the most often heard piece in piano literature.
He was famous for composing German classical music of the Romantic era. His most successful works have been his songs for voice and piano, the A minor piano concerto, two piano sonatas, and various character pieces for solo piano. He was also an influential music critic.
Depending on where you are from, the most popular songs by Mozart in America are; the first movement of Eine Kleine Nachtmusik, Symphony No. 40, Piano Concerto No. 21, and Piano Sonata in A Major K. 331.