Obviously, the MCAT is a chance to test your basic science knowledge. You should have a thorough understanding of Bio, Chem, Organic, and Physics in order to do well.
With regard to long-term value, Genetics, Cell Biology, Microbiology, Physiology, Embryology, and Biochem are probably the most useful when it comes to actual medical school. Although these don't matter too much in admissions (except as upper-level prereqs dependent on school), it's always good to gain first exposure to them in undergrad to make the transition to med school easier.
These subjects are generally taught in your first year, and will be tested on Step I. Having some knowledge going into each of these classes will make life a lot less stressful in the long run, trust me.