I'm just about to learn the Pathetiqué, but is the Hammerklavier the most difficult technically, or musically? Or both?
Hammer Klavier is pretty much as scary as pieces come; It's close to the very top of my list for being scary (beating out Liszt's Don Juan, Ravel's Gaspard, many godowsky etudes,).
Technically
sure there's a LOT of notes, but even worse is this sonata is extremely unpianistic. I doubt the notes would ever 'fall into ones fingers'; it's obvious Beethoven wrote this without any consideration for the pianist whatsoever. No matter how many thousands of hours spent practicing it, Hammer Klavier is never 'safe'. The fugue section in the last movement is just... well, unplayable I guess (though obviously there have been people who have pulled it off). Also, the piece doesn't even sound that hard, and it's just about the hardest piece ever (figure of speech here).
Musically
there are 2 types of musical complexity I will address here; emotional complexity, and intellectual complexity.
Intellectual complexity is the complexity of the score; the structure of a piece, the musicaly reasons for some of the radical things composers did, etc.
Emotional complexity comes from the emotional termoil that the composer was feeling/wants the pianist to feel when playing, and what the pianist wants the audience to feel. It is the understanding of the piece outside the printed notes. Hammer Klavier is quite intellectually complex, but I personally don't think its too bad (I think the liszt sonata is more intelectually complex for instance). On the other hand, I haven't heard a piece that was close to as emotionally complex as this. Some of Beethoven's ideas are so weird and confusing but heart-wrenchingly beautiful. The fugue in the last movement is grand in its highest form, yet it is so alien. Though Hammer Klavier is nearly unplayable technically, I think the largest obstacle to overcome is the emotional complexity it contains.
op. 106 is the monster sonata
this is barely scratching the surface... There are
huge works that don't even come close to this (rach 3, gaspard de la nuit, [insert enormous piece here]). After looking at Hammer Klavier, even the largest works seem insignificant. Here's an interesting story to think about.
Liszt could learn monster pieces in a matter of hours; he could sight-read chopin etudes and never forget them, he could play entire orchestral scores just after flipping through the music. How long did it take him to learn Hammer Klavier?
MONTHSI think this should be scary enough to convince 98% of pianists to not even try.