Dermatology. I believe this is why dermatologists have the highest career satisfaction of any group of docs, when various organizations have done such surveys. It's a combination of the money being good and the stress being low. Of course, they do do procedures and that comes with a certain amount of stress, but my impression is if they are not sure about something, they can just biopsy it. Also, they are some of the few doctors who can still go into private practice.
People mentioned psych, ophtho and anesthesia. Psych can be stressful because patients get assaultive, homicidal or suicidal. Ophtho - you can blind the patient if you screw up...so I would say NOT low stress...at least on some occasions. Anesthesia...you can quickly kill the patient if you screw up...airway emergencies, drug rxns, etc. plus potentially getting yelled at by surgeons...and maybe a CRNA will take your job or you'll be forced to "supervise" one...so I would say NOT low stress, though anesthesia jobs vary in their level of stressfulness (I'd say CT anesthesia >> stress of working at an outpatient surgery center than staffs mostly minor surgery cases).