Great! I'm very happy to know that!
Thank you so much Pierre, it's really clear and will help me a lot!
Thank you for your question!
It is very difficult to answer this question because it depends a lot on the horse I'm riding!
But if I have to choose a particular exercise, I would say the transitions, especially the trot-walk and the trot-canter transitions.
Both are great to improve their balance and also their responsiveness. But they also help horses to understand quickly to respond to very light aids, and also to find the best way to use their body. These transitions also help the horse to integrate immediately the basics that they will need for the rest of their life, no matter their future discipline.
Otherwise, I really like to have young horses a little bit too fast in the trot in the early stage of their training. From my experience, it helps them to understand immediately how to think forward. When they are used to give all their heart in the forward motion, it is very easy to slow them down to create a good swinging gait but keeping them with the will to go. And you will never have to push them to go forward, just to let them go, which is super confortable to ride!
Of course, I want them to keep a cool mind when I do that.
I also pay a lot of attention on the natural crookedness from the start because it is so important and, in my opinion, it is the cause of 90% of the behavioural and/or physical problems we might face later on.
I hope this answers your question!