I would say the hardest pieces (overall) I have learned would be Beethoven Op.111 and Op.57, Chopin Ballade No.4, and the Corsellis arrangement of Bach's Passacaglia and Fugue in C minor (this one is an absolute beast).
Currently I am working on the Liszt Dante Sonata and that is really challenging, but it is still so much fun to work on. I am also working on Vers la Flamme by Scriabin, starting at the end, and, while difficult, I actually don't think this will be too terrible in comparison to other things I have worked on or learned before.
Despite any of these so called hardest pieces I have learned, I think I have learned more difficult pieces for me at the time I was learning them (on my road towards where I am today). For example, when I was much younger, I had gone through a phase where I wanted to quit and was struggling with growing up, playing sports, and all of the things that happen when you are a very young teenager. I had just lost interest in the pieces I was learning, wasn't exploring a lot of music, and just was kind of ready to move on. My teacher saw this, and we began working on other pieces to try and keep my interest intact. It was only somewhat effective, so my teacher challenged me and pushed me to learn the first movement of the Pathetique sonata by Beethoven. I was good enough to learn it, but only if I really devoted myself. It was quite difficult for me and I had to practice a lot for a few months. It taught me how to practice and showed me I could learn difficult things. It increased my appreciation for music and that was literally the piece that pushed my to be obsession. Moral is, that was definitely the hardest piece I ever learned, even though today I could pretty easily breeze through it.
-KC