For me it's definitely teaching. Whether I teach coding or any mathematical or even theoretical subjects. I find that when you teach someone you learn how to communicate better and transfer knowledge effectively. Communication is key in client relationships.

Secondly when you teach someone a concept you think you understand you tend to find flaws in the way you understand that subject matter by forcing you to hear your explanation out loud. This in turn will make you delve deeper into that subject matter and make you understand it better, rearranging your own perceptions and correcting those flaws.