the bagpipes, hands down.

Any instrument I dont care wich in the altisimo register or the lowest notes is hard to play and forget mastering it, except for the piano and strings. Also have you considered that there is another factor (speed) how fast can you play any instrument, playing a fast clarinet or guitar (new orleans style)or reading both parts for the piano in a fast clasical piece?

didn't get time to read all of the replies, but I'd say the bagpipes are hard to play - I've got a full set and can't get a note out of them yet....!

I play the clarinet, trumpet, guitar and piano out of all those instruments the piano has been the hardest to play well, anyone can press keys on the piano or build triads and say they play but that's not enough to conclude that it's easy, apply that to all instruments.
Anyways It really gets to me when people who don't play piano but play guitar say that guitar is harder than piano, I think thats crazy. What do you guys think?

The French horn is definetely one of the hardest instruments to play. I've played it for almost 7 years, which isn't that long, but it takes a lot of practice. I've also played the piano for 9 years, and it's one of the easiest instruments to play.