I learned the latter two movements of the B flat minor sonata, op. 35 (Funeral march and Graveyard movements), and found them to be adequately challenging, but the real technical challenge is getting the "hammerless" effect to make the synchronized voices sounds like a wind swirling through a cemetary.
I think the Chopin Polonaises pose the most technical difficulties as a whole, and out of his fifteen polonaises, I found 7-8 of them very very challenging. The B minor sonata is very difficult also, but don't underestimate the first movement. That is no easy piece!
I still vote for his polonaises/b minor sonata/10/2/25/6/scherzo 1