In fact, I think, it's depends on the person, who is asked this question. A person who is suffering from cancer might say
that the greatest invention would be the medicine that heals them. A person, who can't walk, might say it's the wheelchair, that's helping them to reach their destinations. Or a person who has a
broken leg thinks it's the plaster cast that helps the leg healing by holding it firm.
For me, it's the discovery of bacteria and the invention of antibiotics like penicillin. With that, people haven't to die in result of an illness that's caused of bacteria.
Before germs were discovered, a lot of women died because they became ill during birth. Doctors didn't clean their hands before helping the baby come to the world. So the bacteria went into the organism of the mother and made her seriously ill, sometimes so bad that she died. The illness named childbed fever was the main cause of mothers' dying. After birth the bacteria went into the body and increased. So the mother got high fever and sometimes she died. After discovering the case, doctors had to clean their hands with alcohol so they were free of bacteria and less mothers became ill.
The invention of antibiotics has helped a lot of mothers to be healthy again. In these times there are a lot of antibiotics which help the people. Sometimes they are taken too often, so the bacteria become resistant and they can't help no more. So it's important to use them responsibly.