I don't know enough about the various degrees to comment on each one's marketability, though engineering fields are generally pretty good for that.
As someone who planned for a long time to ultimately attend medical school, I'll second the biochemistry recommendation for preparation. Just using the local university and med school as an example, the biochemistry degree fills all the med school's requirements (specifically: a year each of English, chemistry, organic chemistry, biology, physics, and another science), throws in another useful sequence (namely, biochemistry itself), and leaves room for other possibly useful classes (anatomy/physiology, immunology, medical microbiology, endocrinology, etc).
Keep in mind that the MCAT is more a test of reasoning ability than it is a test of knowledge. While extra courses will undoubtedly help, the basic requirements for med school admission should be adequate if all else fails. Also, as I'm sure you realize, make sure you pay attention to things outside of your coursework as well. There are many many students with excellent numbers applying to med school every year, and only something like 50% of applicants are actually accepted. Work on becoming the most well-rounded person you can be.
Good luck, in any case. :)