Calculus by far. I read this "Calculus Made Easy" by Silvanus P. Thompson and it's still to this day my inspiration for explaining complex technical topics to

Calculus by far. I read this "Calculus Made Easy" by Silvanus P. Thompson and it's still to this day my inspiration for explaining complex technical topics to lay people. It's a fantastic book, and even if you know math you must read it if you want to understand how to teach complexity to others.