I will start with the Davy lamp as i think it saved many lives.
The Davy lamp is a safety lamp devised in 1815 by Humphry Davy. It was created for use in coal mines, allowing deep seams to be mined despite the presence of methane and other flammable gases, called firedamp or minedamp.
Although Penicillin wasn't discovered by Florey and his team, i think 1939 is the
1939, Howard Walter Florey and a team of researchers at Oxford University made significant progress in showing Penicillin's in vivo ability to kill infectious bacteria. This eventually led to commercial production of penicillin and the belief that modern medicine has led the world in an era free of diseases.