Internal medicine. I interviewed my girlfriend's aunt who is also an internist and she said, "While specialists know their specialty, I know my patient."
I do think that closely know who your patient is has a large impact on outcomes of treatment.
Not only that, but internal medicine is the largest specialty out there. So there are plenty of residency spots.
In addition, I have heard of a lot of fellowships (If I decide to subspecialize further) would pay for an MPH. That is definitely one of the things I want to do in order to treat the individual, community, and nation as a whole.
Of course if I do the MD/MPH degree as a part of medical school then I will take it then. Otherwise I will do it before/after residency.