Invention of anti-biotics. This invention alone has saved millions of lives and greatly reduced human suffering.
That is total bullshit. Infection is the leading cause of death before antibiotics and is still the leading cause where they are unavailable.
Anyone who has had an infection will tell you that.
I know it's cliched but I'm going to have to go with Internet. That or nuclear power sources, in that we now have world-destroying capabilities that humans haven't really ever held before. But I think I'd probably say internet as that is being more actively used, and with that I also mean the development of computers and high-speed digital technology in general.
Vaccines, by a wide margin