Humans have invented many things and it really depends on each generation what was the most important invention. There are few that come to mind below
1) Fire: Ability to create fire led to significant increase in Humans ability to survive and thrive.
2) Wheel: Ability to create and control wheel led to Humans being able to cover more distances than they previously could do physically. This led to increased ability to hunt and also run away from predators
3) Farming: Ability to Farm food led to Humans not needing to hunt and led to creation of societies that led to more cooperation & more inventions
There are many others like Gun, Engine, Drugs, Electricity, Airplanes, Nuclear energy, Computer, iphone.