In my experience, the teachers I learned the most from were also the best teachers, i.e. the most engaging. Difficulty was really a separate concern, in a lot

In my experience, the teachers I learned the most from were also the best teachers, i.e. the most engaging. Difficulty was really a separate concern, in a lot of ways. There were very easy courses that I learned a huge amount from, and difficult courses that weren't worth the time I spent on them. To be honest, I only bothered to rate professors because I couldn't see my final grades until I did so. Consequently, only the very excellent and the very terrible really got any kind of response.