Of the classes listed, A&P (especially physiology).
There is also alot of genetics since last year which replaced some Organic.
The last poster mentioned Biochem (I found it helpful, but after taking the biochem and then the MCAT, I took my cell biology major class, and feel that it covered everything on the MCAT w/o going into the detail that Biochem invariably does).
The other classes (histo, immun. etc.) all contain nice to know information, but the MCAT doesn't specifically test over information covered ONLY in those classes. However, taking those classes might reinforce knowledge learned in lower level classes which would make your more confident in those subjects.
(Ex. is mitosis). Obviously the MCAT could test over mitosis, so it only stands to reason that the applicant who had taken gen. biology, zoology, botany, cell biology, genetics, biochem, and physiology would likely have it down cold and be able to spend a minimum time on those questions than someone w/ biology and zoo alone.
BTW: Ecology and limnology for the MCAT are essentially worthless. (except for evolution concepts which are covered in zoo)