Assuming you're referring to medical doctors, there is no single answer.
For Example: Dr. Charles Drew, who developed the process of blood separation allowing plasma to be collected and stored over longer periods.
We take that for granted today, but it was a major event. Louis Pasture, discover the need for pasteurizing milk; Dr Jonas Salk and his team for developing the polio vaccine and countless more. Doctors make discoveries that are built on other discoveries and so on down the line. The doctor who saved my life with, I had a perforated colon and did a 90-minute procedure in 45 so, I could get off the anesthesia ranks high on my list. The surgeon who operated on my son who suffered a brain injury following an auto accident also ranks very high.