Gah. I hated Organic Chemistry lab at UofI. I had wanted to take it the same semester as O. Chem II, but due to a scheduling conflict with Physics, I had to take it the next semester.
Normally, O. Chem lab is a pretty easy class. There's an optional lecture, where you show up, learn about the basic background for the upcoming lab, etc. But if you read it ahead of time, it's good enough. Show up to lab twice a week, chemistry the **** out of stuff, write a report on it. Take a couple test on the labs. That's about it.
But unfortunately for me, I ended up with a new professor fresh out of his grad school/post doc at Harvard. And apparently, he thought that Organic chemistry Lab needed to be pretty much Organic Chemistry I and II crammed into one semester. Ended up having two or three exams + final, which had to do less with the lab, and more with memorizing, identifying and drawing out tedious reactions.
Still did well, but I it required actual studying! I've always hated that my friends who took it the semester before had to work about 100x less than I did for the same grade.
In terms of actual difficulty based on material, it was probably Neurobiology for me. Last class I took as a senior for my Neurobiology track. Lots of material, long lectures and a professor who flew through material for 1.5 hours straight.