Rach3 is the hardest Concerto for sure...right next to some of the Bach Concertos, which are the ultimate tests of endurance.
I think some of the hardest songs are from the Godowski - Chopin Etudes. Hmm... Actually, anything transcribed by Godowski.
If you mean technical dificulties, you should really look at the Liszt Paganini etudes BEFORE his Weimar revision. They´re not THAT hard, but the version of those etudes that he wrote when h was young (he very first one) is almost techically imposible.I believe Gaspard de la nuit is EXTREMELY dificult.... bu musically, the hardest are Scarlatti sonatas!