I don't think there is any 1 thing that is more challenging than the other. I find all aspects of dressage challenging, thats why I love it so much, never a dull moment.

With a young or green horse, it takes a lot of time and training to instill the basics - straightness, forwardness, contact, accuracy, calmness etc.

Moving up the levels there is always something that a horse will struggle with. My horse for example finds collection difficult, but he finds medium and extended work very easy.

For my current horse, his brain has been the most challenging thing. He is naturally talented and purpose bred for dressage, but he is so hot. Riding him is definitely a fine art...1 wrong move on the riders part and he is very unhappy. He needs to be ridden very delicately and with much care and sympathy towards his temperament.

If I had to choose the 1 most challenging thing, I would say submission and relaxation, because really, when you have those 2 things (which generally go hand in hand) you have it all. You just have to train hard for the rest to come.