Most undergrad classes are pretty useless to medical students taking the boards, but personally I was very happy to have taken Immunology in college. Our med school immuno course blew goats, and my step 1 had quite a bit of the subject.
Undergrad developmental bio. Not that embryology is a very high-yield topic, but my school's embryo class (or, to be more accurate, lack thereof) didn't hold a candle to the once-a-week class I took at night school three years back.
Immuno as well, although certainly less so. Biochem almost nil.
Immunology
Genetics
Embryology
Vertebrate Physiology (basically a cell/molecular bio course w/ lots of 2nd messengers, etc.)
Histology -- makes 1st and 2nd year much easier
Instead of worry about step 1, it makes much more sense to think about which undergrad classes set you up well for the MCAT. I think biochem and molecular are good for that. Also, if you take the real physics (not the no-calculus' physics that most pre-meds take) you will have a better understanding of the prinicples involved and the physical sciences section of the MCAT will be much easier. And yes, I am aware that there is no calculus on the MCAT.