hardest piano piece i've ever played

Here by hardest I mean mechanical difficulty, not separable with musical difficulty though.
Mechanical difficulty, in my definition here for this blog's sake, is expressed in terms of the time it takes to be able to simply press the keys at a reasonable tempo, and does not mean at the level you are fully comfortable with the playing the piece beautifully in public.
The time it takes to reach that level is decomposed into (i.e. sum of) (1) the time it takes to read the score, and (2) the time it takes to overcome mechanical flaws.
One of the hardest pieces I have ever spent time on is Lavapies from Iberia by Albeniz, whose difficulty is mostly attributed to (1), like other pieces I tried such as 10th gaze by Messiaen.
The other of those, contrasting to Lavapies, is the 1st movement of the piano concerto No.2 by Prokofieff, on which I spent more time on (2).
The piece that I just started considering playing in the next big public performance opportunity is "Reminiscences de Don Juan" by Liszt, which I am going to apply the same strategy and tactics in conquering mechanical difficulties, i.e. rigorous decomposition of the piece into parts, prioritizing them by difficulty setting target # of repetitions, and integrating the parts into whole once I overcome mechanical flaws. By rough estimate, as I played it only a few times so far, is to take at least cumulative 30 hours of practice to give it a shape, and another 30 hours to get it the level I can play it in public, and another 10 hours to bring it to the next stage, i.e. to be competitive in competitions.