In terms of complexity of the actual instrument? I'd say the organ. It's this giant monster of an instrument. In terms of learning, I'd say the violin is the most difficult. You have to be really good to just get the notes to sound right. If you are even slightly off, the sound of an incompetent violin player just sounds awful. On the other hand, I've heard that piano is easy for beginners, but one of the most difficult instruments to master.