Piano Forum

K 576 is probably considered the hardest one. It's hard to say for sure as you might find some aspects of Beethoven or Mozart or specific aspects in each piece easier or harder than other people.

In general I would say it's easier than the major Beethoven sonatas like Waldstein, Appassionata, or his late ones (Op. 101, 106, and 109-111). Perhaps it might be similar to the Hunt Op. 31 No. 3 or A Thérèse Op 78 in terms of difficulty.

Is this your first Mozart piano sonata? If so, I would highly reconsider doing another one before tackling this piece. Did you choose it? I find it hard to see a good teacher assigning this as a first Mozart piano sonata unless the student really was interested in this piece in particular.