I think all of these undergrad classes can be easy with a good professor... and it also depends on how mature your study habits have developed.
Hardest:
Organic Chem I
Hematology
Molecular Biology
The first class to kick me in the ass was my Organic chem class. I can blame it on myself for still having a high school study habit. Then came Hematology... these exam questions came straight from the MCAT question bank.
I'm now in my 5th year and my hardest class is Molecular Biology. The concepts in this class are so easy but the professor manages to make his exams damn near impossible.