1.Liszt
Liszt is sensationally difficult so much so that I give him the one spot, Liszt wrote the transcendental etudes which are some of the most difficult pieces overwritten, espescially no5, Feux Follets, which is absolute hell. Additionally, he has the paganini etudes, the hungarian rhapsody's (check out number 2 and thank me later) and tons of transcriptions, his music is exceptionally difficult. Anyone who can play all the transcendental etudes has mastered the technical aspect of piano playing period.
2.Rachmaninoff
Most famous for difficulty is his 3rd piano concerto in d minor considered to be one of the hardest pieces of all time very long, very difficult.
3.Chopin
One of the greatest piano composer's of all time, he wrote 27 piano etudes all of which refine some aspect of the pianist's technique, particularly difficult are the winter wind etude(I've played that :), op 10. no.4 and perhaps the hardest the thirds etude (I wouldn't even want to touch the thirds etude and I've been playing for a while.)
4.Ravel
Ravel wrote Gaspard De la Nuit (devil of the night) a very very challenging piece, and also considered one of the hardest of all time. I've learned the Ondine section, and I can tell you it was extremely rough. The third section (Scarbo) is even tougher.
5.Beethoven
Beethoven's music is very difficult all though I don't find that it reaches the heights of difficulty achieved by Chopin and Liszt and I don't think he wrote anything as hard as the transcendental etudes, or the hardest of Chopin's etudes. He wrote 32 piano sonatas no. 8 pathetique is one of the easiest one's he wrote and is a good first sonata to start with. no.17, the tempest is pretty difficult, the main challenge is the tremolos in the piece, which are encountered often and can be difficult to execute, you also have the 23rd(I think) piano sonata the appassionata is difficult as well, I find the most difficult aspect of this sonata to be the runs in the 3rd movement and some of the tremolos in the 3rd, you need considerable finger strength to play it. The hardest sonata he wrote is the Hammerklavier sonata, apparently, but I've never looked at it so I can't really comment. All in all, while Beethoven might not have been the hardest of the hard composer's he wasn't far behind them. To play some of pieces he wrote, the way they are meant to be played you need to possess prodigious skill, and perhaps even technical mastery of the instrument. Good luck, with your pianistic pursuits.