This doesn't necessarily mean hardest, but the class that just f*cking sucked while in college, because of waste of time, teachers, etc.
I'm at a toss up, either Engineering Graphics or Engineering Physics 2. graphics is a 2 credit hour class, but you spend 6 hours a week in class, and another 3 or 4 hours on HW.

EP2, well it is pretty hard, but what really grinds my gears is the god damn 430 tests on fridays...and there's 6 of them in the semester.