General/Trauma Surgery, Family/Internal, or Pediatrics are always good... also several organizations look for anesthesiologist to come and help during surgeries (Operation Smile type stuff). What physicians have told me is that if you specialize too much (say cardiology, oncology, resp) you can't do as much.
One oncologist said "so what if I can diagnose them with a cancer I can't treat it" (refering to trips to Central America) but she is able to listen for basic things, it's just she doesn't remember every little thing from medical school general rotations and she doesn't see those issues anymore.