The answer to this question can be quite subjective. Some people will say that the telephone is the most important invention, or the automobile, or the plane, but for me personally, I'd choose antibiotics.
It is a modern-day invention (as opposed to the Middle Ages, for example) which changed our lives for the better. Before penicillin was discovered in the 1920s by Alexander Fleming, people were constantly dying because of most harmless diseases. But after antibiotics were invented, people were given a chance to recover from their illnesses much more easily.