It would have to be the discovery of the antibiotic penicillin by Alexander Fleming (and the subsequent development into a medicine by Howard Florey and Ernst Chain). This was an accidental discovery because he allowed the agar jelly that he grew his bacteria on to get all mouldy (like the mould on bread). He noticed that where the mould grew, the bacteria got killed – so the mould was producing something that killed bacteria, and this was penicillin.
Penicillin arrived at a time when people could die of simple infections, even a cut to the hand if it went yukky, and it went on to save more lives than any other drug in the 20th century; though unfortunately this original form of penicillin is not nearly so useful any more.
I think antibiotics are a pretty amazing scientific discovery, especially as it was pretty much an accident (combined with a great scientific brain!).
However I would like to throw vaccination (usually credited to Edward Jenner, although there is some evidence that the idea was known much earlier) into the ring as a contender for best scientific discovery.
The reason for this is that what Jenner did was formulate a scientific hypothesis: that cowpox, a mild disease similar to smallpox, might protect against this much nastier disease. This was based on the observation that milk maids, who often caught cowpox from the cows they milked rarely got smallpox despite the fact it was very common at the end of the 18th century.
Once he had a hypothesis Jenner tested it by infecting a boy with cowpox, and after the boy had recovered, exposed him to smallpox, which (fortunately) he was immune to. While this sort of experiment would never be allowed today it does show how the scientific method can advance knowledge in a rational way, with huge benefits… smallpox was declared “eradicated” (i.e. destroyed in the wild) in 1979.
Well depends on your definition of science but I would have to say the internet.
I think we take it for granted but it has greatly improved scientific research from getting hold of research materials, finding information and reporting information that you have found through experiments.