The Thermionic Valve by DeForest in 1915 really was the birth of the electronic age. Without this invention most of us would never have been born. Without electronics this planet would not be supporting the massive numbers of people now living in the West. We would not be able to communicate, compute, manufacture and distribute atoms on the scale we now enjoy. There would be no radio, TV, computers, Internet, modern medicine — engineering, international travel of any scale, atomic power and almost everything we currently take for granted. In fact our species and our civilisation would have stalled without this invention.
This Thermionic Valve is very closely followed by the Transistor in 1945 with Bardeen and Shockley creating the foundation for what you are reading this on — the PC.