I was worried about physical sciences. It had been 3 years since gen chem, one quarter of which I skipped because of AP credit. Also I was not done with my physics classes. And I didn't take any prep courses or buy any prep books. While I was taking the test there was a whole passage on circuits and resistors, which I had never seen before. So I guessed on basically a whole passage worth of questions there.
And I still got a 14.
Therefore I conclude that physical sciences is the "hardest" section, with some topic in there being the hardest topic if only because most people that take the MCAT answer many, many of these questions incorrectly.