I think the discovery of antibiotics is the most important.
Before them, 9 out of 10 people died from infectious disease. Now only 1 out of 10. That is why heart disease and cancer are now the leading causes of death.
Modern sanitation also figures huge in that.
Agreed with anesthesia a distant second
It has been, and may continue to be, but at this stage, GMO foods have to be moved to, or near the top of the list.
GMO foods have made it possible for those who are alive and well to stay that way by having affordable foods. Before GMO, many simply died of starvation.
RE: “9 out of 10 people died from infectious disease.”
It’s also important to remember that many of the deaths caused by infection were directly related to malnutrition, unsafe water, hypothermia, and accidental or violent injuries.
Even without antibiotics, the world would be much healthier today because of abundant food and clean water, better and more available shelter, and safer work places, transportation, and homes.