hmm...Chem II was a PITA for me, but it was my own fault since I never went.
Client/Server DB Design w/ Oracle was probably the hardest, because the professor went waaaay past the scope of the course. On the first day, he set his own prereq's for the course--3 courses that were not listed as prereq's in the course catalog. No one in the class had taken all three! I stuck with it, though (because I needed it to graduate that summer) and ended up getting an A.
Circuits was hard for me as well, I don't know why, it seemed simple enough, I could just never do well on the exams...