This is a hard one to answer. I think I have three answers!
1. In terms of human survival / health, I’m inclined to say vaccinations. Without vaccinations we’d still be dying in our millions from the flu, from polio, from measles, etc.
2. In terms of changing human history, I think the printing press transformed the way we share information. Instead of relying on handwritten notes or accounts, or word of mouth, we could now share information much more widely, across countries and continents, and more cheaply and faster. The printing press made information – and thus education – more accessible to the masses.
3. In terms of the human condition, I think the telescope was the greatest invention. It allowed us to understand our true place in the universe – a mere planet orbiting an average star – instead of being at the centre of the universe. Telescopes allow us to understand where we came from, in the most grand and most fundamental sense, and allow us to explore the reaches of the universe. There is such hope and wonder in the stars, at realising that we are so small, and in doing so can teach us to value what we have here on Earth!