Well, I think it really makes a difference if you're trying to pass a class or to get an A in it. From all the math/science selections, I'm guessing most people were just trying to pass. Most humanities/social science courses are easy to pass but getting an A can be nearly impossible.
I don't think anyone can judge the selections of other posters because unless the curriculum and teaching are standardized throughout all universities, one student's experience might have almost nothing to do with another's.
Writing 100 is the freshman seminar everyone took here. But depending on the books you studied and your instructor, it could be either extremely easy or extremely hard. A couple that made my head spin were Psychology of Learning and Honors Calc (Calc I and II compressed into a single semester and taught at the honors level). I had to work my tail off for a course on the history of European cities. But loved it - never has anything been so worthwhile. When I combine the amount of work + grading + other stressors...I'd say my hardest course was Composition (music) II. When you spend hours upon hours at the piano needing to get a composition out the next morning and the muse just isn't there...that's about the worst I've ever felt.