Doesn't really matter, you have to get them all done anyway. I studied physical sciences over my winter break because I was gonna take the april MCAT, but didn't end up taking it...so I've forgotten most of it. Now I'm going over Bio....pretty soon I'm gonna get those verbal passages going cause that is my weakest area.
I studied chemistry first and the most often because I took it freshman year and forgot most of it. Then I went to Biology, and then Organic since I took those sophomore year and they were still relatively fresh. Physics I studied last and spent the least time on because I was taking it my junior year, in time for the april test, the one that just passed. Everytime I finished a section though, I would review the other sections I studied right after it. After a while this didnt take much time because everything became more familiar and I could just concentrate on my weak points from each section.