Yes, well, I think all of them are hard :-/
The hardest is probably his second set, (op. 25 no. 6 I think) with the thirds... pain in the neck to play.
Otherwise, maybe op. 10 no. 1, or perhaps even no. 2, because of the extreme care Chopin put into the fingering... if you look at the score, he himself has put fingering for basically every note (except the repeats!) which is quite unusual for a compopser to do.
However, for the hardest etude (generally speaking) every composed, I'd have to say one of the Rach etudes, probably op. 39 no. 1... gosh that's hard.
Anyway, since this is about Chopin I shouldn't be going on about other composers...
Martin
(By the way, I also visited your site and was pretty impressed... I should also start one, one day.... what's the hardest piece you can play?)