Flatwork in general is essential. In just over the past year, I have tried to improve my flatwork and I feel it has really helped my riding. I try to go through a series of flat exercises each time, but if I had to pick one, I would say transitions. Both within a gate, and between the gates. I've found it encourages my horses to relax and submit. Jumping courses require rideability - so this helps me a lot! Thanks for the question!