I saw the "Easiest class at U-M?" post and thought that some might find the opposite question interesting.
My hardest class? "Dynamics of Mechanical Systems" I think it was M.E. 340. That (and M.E. 240, come to think of it) just didn't "click" for me. Interestingly, most of my classmates did not share my opinion. They typically chose Fluids or Heat Transfer.
My easiest? A 100-level (Social) Psychology course. It was interesting and almost laughably easy. I took it during my senior year.