What say the OT?

Fire.

Color tv

Seriously though, the wheel

Airplanes.

The spot lock trolling motor. It's better than sex, and you don't have to cuddle with it afterward.

Seriously, the most important being is God Almighty. The greatest concept is capitalism because it spurs ingenuity, entrepreneurship, effort, and striving for excellence/improvement/success. The single greatest invention is a tough one. I will go with the internal combustion engine because it made formerly impossible tasks possible and gave people freedom of movement across long distances.

Internet

Depends on the day. Antilock brakes in Sturgis SD last Sunday was nice. Too many wheels there.

Soap and water is nice, but somehow not everyone knows about it yet.

Air conditioning

A toss up between fire and the wheel.

Opportunity and empowerment of women.
Declassifying them as property and providing them with expression of mental utility.
Expands your civilization's problem solving ability and potential for innovation by a significant margin; instead of excluding half your population from potentially genius idea development.

You look at the most backwards countries in the world and women are still classified as property.
Some may point to Saudi's wealth as a contradiction to the rule. They have natural resource wealth and purchase other's innovations. Nothing innovative comes from that region of the world.

Coffee cup

Air conditioning, the internet, condoms. not sure of the order but pretty important stuff..

Electricity

Disposable douche has rank somewhere in the top ten...