The atomic theory of matter, because It is so foundational to the technology of the modern world. It allowed the development of modern chemistry, and with it pharmaceuticals, chemical engineering (e.g. the Haber-Bosch process), electronics/semiconductors, genomics, medical science, and more. It’s been an enormous force-multiplier to improve technologies we already had by providing prospective insight into how they work, so we can do better than iterative trial and error. Virtually everything that we touch or use in a day, from coffee to clothing to computers, was enabled or improved through knowledge of the system of atomic elements.