Who are the best teachers?

There are plenty of high quality courses and famous (and not) names, but if you had to pick just one instructor... Who would you say is the best at teaching, and what did they teach you about?

The only criterion is that the teaching style left you the most impressed in the time you had. All levels and related fields are valid, from intro calculus to specialized topics like optimization. Just the same as with the topics, any medium is legit, from a blog post or a short video course, to six years with a PhD advisor.