I nominate waterworks — the system of plumbing and sewers that gets clean water to us and dirty water away from us. I'm hard pressed to think of any other single invention that has stopped so much disease and death. It may not inspire quite the intellectual awe as something like a quantum computer, but the sheer heft of the benefits it brings about so simply makes it all the more impressive. John Snow didn't need to sequence the Vibrio cholerae genome to stop people from dying in London in 1854 — he didn't even know what V. cholerae was — but a pattern of deaths showed him that to stop a cholera outbreak all he needed to do was shut down a fouled well. Without waterworks, the crowded conditions of the modern world would be utterly insupportable — and you only have to go to a poor city without clean water to see this. Another sign of the importance of an invention is the havoc it can wreak, and waterworks score here again—by cutting down infant mortality they help fuel the population explosion, and they also let places like Las Vegas suck the surrounding land dry.
I'd even go so far as to put the importance of the invention of waterworks on an evolutionary scale with things such as language. For hundreds of millions of years, life on land has been crafting new ways to extract and hold onto water. With plumbing, however, you don't go to the water — the water comes to you.