Loved A&P, Chemistry, and Microbiology. Must say the toughest was microbiology. Took it during the summer, along with an additional class, it was a 5-6 hour course. Got out of night class sometimes at 10:30 pm.
Even though I loved it, A&P was the hardest. It was actually the first class that I took and is the basis for all of your nursing classes, so there is a ton of information to learn. I studied everyday for hours.
Chem. Mainly because I found it boring. Sadly, even Organic Chem was more interesting that Gen Chem! (I have a previous BS in Biology with a minor in Chem so I have taken a lot of Chem. Currently working on a BSN). It was hard for me to want to study for something I found so uninteresting. Hopefully you won't be like me in that way! I was able to pull off a B though. Some of the Bio pre-reqs are challenging, but at least I found them interesting. Much easier to study for something when you have a genuine interest in the subject. I have always loved Biology.
Pharmacology was the toughest pre-req for me. The volume of reading plus quizzes & med math minimums of 90% or better made it a challenge. I was taking 17 credits that semester and Pharm ate up the majority of my time.