the hardest i play is fantasy-impromptu,chopin and the flight of the bumble bee-corsakov(arranged for piano by rachmaninov);the hardest i know is 3rd concert of rachmaninov
"Flight of the Bumblebee" is by Beethoven. Anyways, the hardest song for piano ever written which I'd say is impossible to play up to speed is Turkish March by Beethoven!!
The piano Turkish March was Mozart, and yes it is difficult. Beethoven wrote the orchestral one, which also sounds cool. I think the hardest for me is Mozarts sonata in C. It's really fast. For me anyway. A nice easy one is Song of India.
It's hard to say which is most complicated, like hard to say, which composer is best. I'd say Chopin's black note study, ballade in g, etude nr.1 op 10, heroic polonaise, Liszt Hungarian rhapsody and un sospiro. But there are many other hard pieces, for example Rach's, some of them you can't play just because of physical reasons (small hands). Turkish and impromptu op66 are not so complicated compared to above.
None of my pieces are as complicated as any of yours, but after almost a year of lessons, I've learned Schubert's Impropmtu No. 2 in A-flat (7 and a half minutes of sheer beauty), started a solo piano reduction of Grieg's Piano Concerto in A-minor (not as complicated as the orchestral version), and I tried teaching myself some pieces from Mussorgsky's Pictures from an Exhibition (how Mussorgsky put this together so masterfully, I don't think anyone will ever know). And, if I may say so, sometimes complexity doesn't make the piece "better" or "worthy", but often-times, a piece that has a simplistc melody that can be played by sight can achieve admiration from anyone (if you don't get what I mean, try playing Edward MacDowell's To a Wild Rose).
The hardest I have learned was probably Doctor Gradus ad Parnassum by Debussy and Chopin Grande Valse Brillante 3.
Doctor Grad isn't really a *hard* piece but my interpretation/teacher's interpretation made it really hard to play. I probably played it at a pace as fast as Flight of the Bumblebee. (No, I'm not using a brag-like tone. I'm just stating as a fact. Just clarifying for you paranoid people. Sorry) And everytime I play it, I get so tired at the end. But now that it's a year fronm playing it...no sweat. I remembered the pain learning it though. For about 2 months, I had to play slowly but with fast action and power...Yeah...my teacher was definitely nit-picky. But the other hard part was not only just playing the notes in the right rhythm but also the fact that you must add an impressionistic theme in it. How you must paint a picture in notes to make other audience actually *see* the picture you are showing off as they here the music. I thought of Doctor Grad as a picture of snow and calm but powerful wind as well as church bells tolling...This was one of my favorite pieces.
Grand Valse brillante 3. this is probably, technically-wise, the easiest out of all the Grand Valse Brillantes but it definitely needs lots of expression. No matter how much you like it, it is hard to use flow, movement, and expression for me so even the audience can hear it. Also, you tend to lose a lot of concentration because you repeat like 4 really slow parts of the song *AGAIN*. since the whole piece was fairly slow, it's hard to really get into it and actually express without getting choppy...I don't know about you guys, but it was probably the hardest I've learned so far in terms of expression and musicality.
"Flight of the Bumblebee" is not by Beethoven, it is by Rimsky Kavoraz or something like that. The hardest piece for me to learn...which I'm learning right now, is Sonata Quasi Una Fantasia or something like that by Beethoven. It is a very fast piece from the 7th symphony, right after moonlight sonata and that little piece after it.
hmm, hardest I've come across... I would probably have to say Jeux d'Eau by Maurice Ravel. Some other toughies would include La Campanella by Liszt (actually, lots of Liszt is pretty tough, though practice is truly your friend). It's issue is hand jumping that cannot be avoided and probably should not be attempted until you have very strong fingers and have developed good technique for trilling with your 4-5 fingers. If you are looking for a tough piece to play that sounds impressive, I'd go with one I worked on a few years ago, The Winterwind by Chopin. Lots of practice should pay off for you. Also by Chopin, I think someone mentioned his Polonaise Heroic. For a good classical sound, try the first movement of Beethoven's Sonata Pathetique (far from pathetic if you ask me, but only bad if you can't tremolo). Oh, and whoever said Flight of the Bumblebee is by Beethoven is an idiot. Also, a piece I particularly like is the Khachaturian Toccata, but is quite tough and requires development of technique. Islamey is also very respectable.
The fact is that there are very few pieces that practice cannot make easy. Jeux d'Eau is an exception. There are rapid somewhat obnoxious parts that just cannot be done easily. I would probably call it the toughest.