Without a doubt, the hardest class I've ever taken was Zoology. 3 credits, 3 hours of lecture/week, and 5 hours of lab/week. Lecture tests were a combination of short answer and essay, and generally took 2 to 3 hours to complete.
In the lab portion, we were basically handed a scalpel and a dead animal on the way through the door, and were told to dissect it. There was almost no instruction during the lab, and the only reference material we had were the vague descriptions of what to look for from the textbook. The lab tests (every 3 weeks) consisted of several half-dissected animals, with 30 to 50 internal structures (total, not on each specimen) marked that we had to identify from memory. Spelling errors counted for zero credit.
I would have to say that I've forgotten more from Zoology than I've learned from any other class.