I have different favorite exercises to school for each of the boys, since they are so different in their way of going. Phoenix used to be your typical TB; quick, sensative, nervous while Buddy is lazy and slow.
For Phoenix I love schooling lateral work, with lots of changes of direction and bend. Leg yields, figure 8s, and circles used to be our best friends. For a while I couldn't take him down the long side (especially in the canter) without it feeling like he was going to take off. He also used to be pretty heavy on my right leg and rein, so we did a lot to get him to pick up his shoulder and move off of my right aids. Once he was more schooled, whenever he would get worked up and felt hot, a few minutes of leg yielding and figure 8s would calm him back down. It was pressing a reset button. I've been working with Anna on this, and she's just starting to figure it out.
For Buddy, I really like an exercise my old trainer from high school had us do. She called it the momentum pattern and its basically a hunter course without jumps. You start by trotting across the diagonal the pick up the canter and go down the long side. Come back to the trot along the short wall and trot the diagonal. Pick up the canter again and repete. It helps build impulsion and momentum in both the trot and canter, plus it's a good exercise for working on transitions. It really seems to help build a nice flow and rhythm for us.
My last favorite exercise is more for me than the horses. Its also from my old trainer. Instead of posting up-down, you post up-up-down. It works your balance as well as strengthening your two point muscles. I've been doing this when I ride Bud for the two point challenge. Today we hit 4:09!