I agree with Fupila. Organs are some of the most complex, not to mention largest, instruments out there. I'm also agreeing with her about novice violin players are downright insufferable to be around (I'm still haunted by the few months my brother attempted to learn the violin). The French Horn seems very difficult to learn - that's what I've been told anyways. I think the organ probably wins for me overall; complex in a physical way and I believe difficult to learn as well.
Now that you mention it, an organ could be even more complex than a piano to play since you sometimes use your feet as well to play a keyboard on the floor.