I like to work a lot on my basics and keeping my horse as straight as possible. In trot my two favorite exercises are:
1. Shoulder-in, it helps me to bring the inside hindleg under the point of weight of my horse and to increase the shoulder freedom.
2. Leg yielding, helps me get a good contact on my outside rein as well as working on the suppleness of my horse.
In canter my favorite exercise in counter canter. I do this to work on the balance of my horse, it also works on its strength, helps also with collection and engagement.