All things being equal (i.e., you're equally interested/disinterested in all), micro will be most helpful for med school. I'm a big neurobio fan, but that will come in handy only for neuro during 1st or 2nd year and maybe neuro rotation (but let's face it, you'll have forgotten it all by then), genetics you can learn easily along with the course in med school and once you get beyond that all you'll need to know is the bare essentials (likely far less than what you'd learn in this course), however I enjoyed genetics in UG. Cell bio you got plenty of in your pre reqs and you'll get plenty more in first year. Immuno you'll take a course in and and keep up fine--it's another one you'll just need the basics from once you move forward. Micro, however, is a bit of a beast--I'm a third year and it's still one of my weakest areas--and it comes up again and again and again. It's pure memorization. I find it rather boring, which is why I refuse to cram it in, and it haunts me almost daily. If you're interested in it and don't mind some good old fashioned rote memorization, then go for it, might lay a nice foundation for med school.
That said, of course, do what interests you. You can learn everything in med school from scratch, no problem. Of the ones you listed, I would go with neurobio, human genetics, or immunology, but that's just personal interests...
