Teaching
During my time as teaching assistant, lecturer, and finally professor, I've given a lot of courses - in the beginning, mostly exercise classes for other people's lectures, later then lectures based on other people's materials, and finally entire lectures I created myself from the ground up.
The last category is obviously the one I'm proudest of, and as I firmly believe that publicly-funded researchers have an obligation to give their results back to the public, I'm making all these from-scratch lectures available under Creative Commons licenses, both the lecture materials (slides/syllabus) as well as the most recent video recordings of these lectures (thanks, COVID).