Don't really have a favorite. Maybe serratus if I had to choose.
Anytime I get the feeling of having mastery over use of a particular muscle at the same time having it be strong (goes hand in hand), I enjoy that feeling. Once you have full control over a muscle you can flex it individually without having to move any part of your body, and then feeling that muscle be completely firm when flexed. Being totally aware of a muscle while doing a lift that is supposed to work that muscle, and being able to single out that muscle alone to perform most or all of the work during the exercise. For example being able to flex lats to perform pull-ups, and performing the pull-ups in a smooth motion.