The most famous are two grandious piano works, which are somewhat similar to each other: - Prélude, choral & fugue- Prélude, aria & finalThese are wonderful works which should be played more often. (I haven't myself, though)
I love his PC&F. Years ago I did a "cost benefit analysis," and decided they are lots of fun to listen to, but not for me to learn - too many 10ths.
Not a piano work, but his violin sonata
As a pianist and an organist (the organ being, as Beethoven once said, the "King of Instruments"), I have truly enjoyed playing some of Franck's works for the organ. I have played just a few portions of the "Prelude, Choral and Fugue" for the piano--a wonderful work that ends with a ray of sunshine. Franck is one of the underrated masters. You all must hear his "Grand Piece Symphonique" for organ--a masterwork of power and beauty. I have a cd of piano transcriptions of his three outstanding chorals for the organ---but these works sound rather weak on the piano and lack the color of the organ.