Biology.
It's actually much, much harder than it seems like it would be. Biology obeys nobody's logical rules, it's unpredictable, and beyond that, it's not even really a discipline... it's interdisciplinary to the point of being almost impossible to understand to any meaningful degree without knowing all of the other physical sciences very very well.
If I didn't have to take the MCATS, I would switch to being chemistry major. For some reason chem is much easier for me, even though I was sure it would be the hardest subject I would have to deal with this time around. It's the most fun and straightforward and lab is the best part of my week.
Anyway, I have a whole new appreciation for what biologists do. I used to think that shit was the easy part of science, like the sociology department of the hard sciences. My illusions have been shattered!