The greatest contribution to science is political freedom. We don't have to dig too far in history to discover many scientific greats who were persecuted for not thinking like others and for asking questions that "crossed" authorities such as churches and political rule.
We of course immediately always think of Galileo when we think of persecution in the name of science; however there were and will be others. If one reads the history of the early American women accused of witchcraft, it isn't hard to figure out that many of them were practicing herbal medicine or "physick". How about Servetus who was burned at the stake?!? Yikes! And of course everyone's favorite...Einstein who, after Hilter came to power and had a public burning of his books, never stepped foot on German soil again.
I just wonder how many other "greats" there would have been if the fear of persecution did not loom over their head? Those mentioned above are some of the few that carried on in the face of dire consequences.
And then we turn to scientists who made significant contributions but perhaps would not have had their voice heard had it been a different time or different place. Darwin, anyone? No his ideas were not popular at the time, but had it been an earlier time or different place...let's say Galileo's time and place...how would Darwin have faired? Would he have been brave enough to make his findings public?
By far the greatest thing we can do to contribute to the continual growth and expanse of science is to ensure the safety of scientific work within the ethical and moral parameters.
Rather than pinpoint a single scientific accomplishment, such as splitting the atom or discovering the blood flow in the body or determining the origin of the stars, I think the greatest accomplishment of science is the “scientific method,” the system of logic, cause/effect relationships, experimentation toward a hypothesis, etc. that make science work, the connection of the physical world to the mental constructs of the human mind. For that reason, the real central contribution of science was made by such geniuses as Archimedes, Descartes, Newton, DaVinci, the unnamed inventors/discoverers of mathematics, Charles Darwin, Boyle, and the like – those minds who sorted the world out by observation and experimentation. Our relatively comfortable understanding of how the world works is the greatest contribution that science has given humanity.