That's a tricky question for me, being a big Schumann-fan.I'd say his piano concerto and the Davidsbündlertänze are my favourites. Carnaval is epic too, just as the Fantasy and the Geistervariationen by the way. Unfortunately many of his works are very difficult to perform. Personally I believe he ranks with Beethoven, Chopin, Alkan, and Liszt, amongst the big 5 of piano composers. Too bad he never got to perform any of his own works written past 1832, due to his right hand injury...