For me, mechanically the second sonata is the hardest. There are difficult jumps in Op. 17 and Paganini in Op. 9, but they are isolated. Op. 22 is all hard.
I don't find Kreisleriana difficult at all. I think Op. 12, Op. 6 and Op. 9 are each more difficult than Op. 16.
From an interpretation point of view, I think the Noveletten Op. 21 (all pieces as a cycle) is the hardest.