I don’t think we can accurately answer this because thousands of years ago plants were first used as healing agents and although a lot of these plants didn’t heal anyone a few did! But unfortunately we cant say when and where these were first used.
The ancient Egyptians had a system of medicine that was very advanced for its time. Although Egyptian medicine was mostly based on supernatural it developed the concepts of diagnosis, prognosis, and medical examination which influenced how we practice medicine today!
I would say the best type of medicine developed was antibiotics. Penicillin antibiotics were the first drugs that were effective against many previously serious diseases, such as syphilis, and infections caused by staphylococci and streptococci. Millions of people died of infections before this drug was developed in the 1950s and lots of people in science think its one of the best drugs ever discovered. In fact penicillins are still used today!