As a student of history, my hardest classes were the ones where the instructors made the material uninteresting. I often had a more rewarding experience with reading assignments then time actually spent in the class. Many of my undergrad professors simply rehashed what I had read in the text before class.
My Computer Science 415 class was probably the hardest class I've ever taken. Object-based programming in Java. It's really fun when you know what you're doing, but boy did I get more and more lost toward the end haha