Oh, I wasn't really interested in valuing the music on an aesthetic basis. Obviously, most people aren't interested in it. Those just happen to be the most technically difficult pieces I'm aware of, with some other examples from those, particular composers also topping the list. George Flynn's Trinity might not be quite as difficult as some of the others, as Alistair pointed out, and the work by Downie is also very specific in its difficulty; there is a thread about that piece elsewhere on the forum, and anyone interested in seeing why it's a bit of an oddball in terms of establishing a "difficulty" can look it up. And, I don't speak to Powell (not saying that as in, "I refuse to speak to Jonathan Powell for some reason"; just saying that I've only talked to him briefly, once or twice), so I don't know what his opinion is.
I'm not really too familiar with what X, Y, Z pianist has said is the most difficult work, so I can't really answer the precise question posed by the thread. I can only answer what I happen to know/believe (I don't really think there's much subjectivity to the question of technical difficulty, although there are always idiots people who want to chime in about how a Mozart Sonata is so difficult because they believe a performance of it subjectively requires some tangential, such-and-such criterion). Discounting the works above presents a slippery slope: for instance, I might want to mention the difficulty in Bartok's Concerto No. 2 or Ginastera's Concerto No. 1, but someone who has an even more narrow taste might claim those are too challenging, and then we get relegated into the Romantic Era etc. etc. until we're not really answering the question any more. As far as pieces in the standard repertoire (or near-standard), Barber's Sonata, Scriabin's Sonatas Nos. 7 and 8 and Vers la Flamme, Ginastera's Concerto No. 1, Bartok's Concerto No. 2, Prokofiev's Concerto No. 2 and Toccata, Rachmaninov's Sonata No. 1, Brahms' Concerto No. 2, Rzewski's North American Ballad No. 4, Ravel's Gaspard de la Nuit, Stravinsky's Trois Mouvements de Petrouchka, Corigliano's Etude Fantasy, the Tchaikovsky-Pletnev Nutcracker Suite and Liszt's Etudes and Don Juan Fantasy are probably the most technically challenging. But even then, there is plenty of more obscure, Romantic repertoire that is infinitely more difficult: works by Reger, Henselt, Liszt, Busoni, Godowsky, Alkan and Mereaux, among others, which surely includes composers nobody here is even familiar with.
I mean, I think the aim of the actual question, if I'm not mistaken (and let me know if I am), is to find out what the most difficult pieces in the standard repertoire are, from extremely qualified sources ("famous pianists"). I think the list above is a long, albeit entirely accurate one; it might be missing a piece or two that slipped my mind (Hammerklavier, Rach 3, Prok Concerto 5 and Sonata 6/7, and Brahms' Variations did not slip my mind, just to be clear; they're simply not quite as difficult, IMO). I think there was a thread made (even by me, if I'm not mistaken) several years ago on here regarding the most difficult pieces in the standard repertoire. Got a lot of people talking; you'll probably get more information from that than from a couple blurbs about what a certain pianist, whose repertoire is only so large, considers the hardest piece s/he's actually *played*.