Surprisingly, there is no best pre-med major for MD PhD programs, as medical schools do not have requirements for majors. Students are encouraged to pursue their passions and unique interests, even if that means majoring in non-science subjects (such as the liberal arts or the social sciences). That said, the most popular undergrad majors are in the biological sciences.
Whether you choose to major in the sciences or not, the only requirement you must fulfill is completing all prerequisite coursework. Every medical school has its own prerequisite coursework requirements, so verify that you are on track to complete all coursework, including labs. If you are unsure or need help with building your course schedule, speak with a pre-health advisor.