I don't know if such polls are common here since I'm new to this website, but my opinion is that, to even be able to categorize which pieces could be among the harderst pieces ever composed to piano, one should carefully think the definitions and requirements of a "hard and difficult" piece. What makes them to be that hard and difficult for pianists in general. It also depends how much these different characteristics of difficulty is emphasized and it should be noticed it's hard to be really objective since the the definition of difficult is often a subjective experience too, depending on the skills of the player. It's a matter of taste too. Which makes this question also a bit questionable and challenging. One can always make the definition of difficult/hard piano piece in general level by general aspects.
Some people find some other pieces harder than others, for different reasons. What is difficult? To make the sound quality perfect? The technical difficulties? The need of stamina because the massive lenght of the composition (like Alkan: Concerto for solo piano)? The format and style of the composition? All these? Something else?
I think the pieces you listed may be among the hardest pieces, but it's a bit constricted list too, for the reasons given above in my opinion. It would be impossible to do an objective, comprsehensive full list about the world's most difficult piano pieces, but it is an interesting question of definitions too. Good luck for you to find answers, I look for the results too since it's very interesting to find out which pieces people consider to be difficult ones.
Poll
Topic: Hardest Piece of Solo Piano Music Ever