I nominate Hummel Sonata no. 6 in D, Op. 106 for the hardest piece from the classical era. It was written 6 years after the Hammerklavier (what a coincidence with the opus number). On the surface the style is very unpretentious, but there are a lot of finger acrobatics involved. For example: the development of the first movement has a passage with parallel thirds for both hands, crossing! The last movement, which starts out like a music box, evolved into a quite a beast. There is a passage with running parallel thirds for the left hand, which imitates the right hand in the preceeding passage.