Anything from the hand-axe to the quantum computer.
Wow, that's a good question, I need to think about it.
Electricity. Without it you'd have no modern society.
Deliberative Thought ie Why?, The first questions. Without that, no stone-axe or flint knives. Other animals use objects as tools. However there is a tremendous gulf between a stone being used like an axe and a stone deliberately shaped into even the most primative axe. The sheer number of perceptual and cognitive efforts required is staggering. I know it is difficult for most people to wrap their heads around, but you need thought tools for thought experiments, and thought experiments are prerequisites for effecting change in one's ecology socially and materially.
I'd vote for the ability to make fire, when and where it was needed; then there's flint knapping, the beginnings of Science, investigating the 'grain in the stone.'
The neocortex, invented by Mother Nature, seems to have started our rise as dominant species on the Earth, and gave us the capacity for doing both art and science.
Which was not a human invention.