The airplane. It still amazes me every time I take off and the plane doesn’t drop out of the sky like a stone.
It’s a horse race between writing and agriculture. Agriculture (and/or herding, depending on if you want to lump them into “farming” or something like that) allowed for population density, large social groups, development of non-food-producing specialists, and protection from famine.
Yes! What else would we have to compare new inventions to…
“It’s the greatest thing since…since…aww just throw it away.”
The thermos. It keeps hot things hot, or cold things cold. How does it know?!?
The same way a toaster knows when brown bread is done.
The birth control pill. Duh.
Screw that. 
Without writing, we wouldn’t be able to get the prescription in the first place.
Let us resist the urge to pick the thing that laid the foundation for other things. If you need X to get Y, it doesn’t mean X is greater. Barry Bond’s parents weren’t the greatest baseball players ever.
That being said, agriculture is the greatest invention of all time. Not because it makes all else possible, but because it single-handedly diverted the direction of the human race to a wider degree than anything else. The greatest modern advancement is plumbing.
The spear?
Soap, to quote Donnie Darko.
Why?
Cant be because I have heard of tonnes of stuff that are the greatest thing since Sliced Bread… It is a bench mark though
Without weapons, we’re just squishy little creatures who get torn apart up close by animals; with spears, we – hunt 'em down as food, in between bouts of spearfishing, readily dealing out death from a safe distance. What good is agriculture or writing if you don’t have something pointy on a stick to keep the other animals at bay?
We survived as illiterate hunter-gatherers back when. How?
I wanted so much to be the first to say this!
So what, then… Grand Theft Auto III?
FIRE!!!
OK, maybe planting things and waiting for them to grow.
Oh, wait… I have already suggested this.
Another vote for language. It’s what makes it possible to have collective progress.
If you’re defining “greatest” as “contributing most to bringing humanity to its present state” (as most of the people suggesting writing seem to) I would argue that agriculture (or irrigation, or the plow) is the greatest invention. Without the ability to reliably produce food, and a surplus of food at that, far more people would have to work full time on hunting and gathering food just to keep their families/tribes/clans alive. With a reliable source of food, more people are free to follow other pursuits which leads to advancements in pretty much every field of human endeavor for which humanity simply wouldn’t have had time otherwise.
Yeah, but how about Ron Popeil’s parents?
But his Godfather and mentor, Willie Mayes, was arguably the greatest non-steroid player ever.