Grades are the most important thing when applying to medical school after making sure you take the required classes in your undergraduate program. Visit you college health careers advising center to make sure your program meets the requirements. As far as which of those majors, chemistry will help you stand out among other medical school applicants. It is likely you will change your specialty focus over time (this is normal) so focus on excelling in the basic prerequisites (things you have to take first). Good luck!