Anything from the hand-axe to the quantum computer.
The antimatter toaster - I'm from the future.
What the hell are you toasting that you need antimatter to get it done?
A cell phone. I don't remember how I lived without one.
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.
Electricity. Without it you'd have no modern society.
Chocolate....chocolate coated anything.
Written language. That certainly paved the way for humans to accumulate knowledge on a scale that was never possible before.
Think I gotta go with this.
I have to say the best thing ever created by man (human race), is just one thing, Coffee! 2nd would be bacon.
The Fleshlight, obviously.