The hardest piece I've been able to play reasonably well on the horn probably was the third movement of Schoeck's horn concerto. The hardest piece I've
attempted to play but failed horribly was Ligeti's Trio for violin, horn and piano. Fast, uncomfortable runs, high notes (lots of D's, at least one Eb) and often staying on such a high note for quite a while (in pianissimo).

It's a great piece though.
I've never played something really hard on the piano. Although it's my main instrument, my technique sucks. Luckily there's plenty of piano music that's beautiful and not too hard.
As for general hardness, some of the most frightening ones I've seen were from Ferneyhough (naturally), Stockhausen, and Xenakis (nomos alpha for cello solo).