Dressage. I know I am biased, but I have ridden in other disciplines and have always come back to dressage because I enjoy being challenged.
It is physically demanding to look like you are sitting absolutely still on an animal trained and bred to move as much as possible. It is physically demanding to train the horse because you must always be straight and in balance, even when the horse is not.
It is mentally demanding because one cannot just hop on a horse and train it to GP dressage. You must understand the progression of training, understand the aids for each movement and know when to push, when to back off, etc..
Despite requiring a lot of physical and mental strength, one must also have finesse and feel to guide the horse, not force him. A rider must know exactly where every footfall of the horse is, without seeing it, and then be able to adjust that!
I've been doing dressage for over 10 years and am still a peon. It takes tens of years to really begin to have a good grasp of dressage and (I feel) to be able to even say that you can coach or train in the discipline.
If I didn't actually know that most discplines were hard I would say reining. But wether you jump,rope,trail ride... You will be challengend along the way. Saying that you can't just go x is hard x is harder x is hardest.
Depends on the horse and depends on the rider. One rider who takes easily to jumping may struggle with western pleasure, one rider who does dressage well may struggle with cutting. A horse that is built for dressage will find it more difficult to compete with/move like a normal WP horse, and vice versa. One cannot say that one discipline is 'harder' than the other unless we are speaking in specific terms as it pertains to the individual.
I would say campdrafting - You have to learn to control TWO independant minds at once - And your horse has to be able to follow your aids precisely enough that you dont' lose the cow, while you are focussing on the cow and not on them.
I think that no discipline is more difficult than the rest. What may be difficult for one, may be child's play for another. For me, jumping is my hardest challenge. I can jump, just not well. I find it difficult to get into the right position. My easiest discipline is probably hack classes (show classes on the flat).