Anything from the hand-axe to the quantum computer.
I have to say the best thing ever created by man (human race), is just one thing, Coffee! 2nd would be bacon.
Chocolate....chocolate coated anything.
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.