My instantaneous response was: Gutenberg's printing press with movable type. This knee-jerk response was followed by a pause and reflection.
What is meant by "invention"? So, to the dictionary! Essentially, anything that did not exist previously, whether it be a mechanical device or art,literature,or music, is an "invention". Sobered by this, I reflected again.
The skulls of l0 skeletons found in Skhul Cave at the foot of Mt. Carmel in Israel in the l930's are similar in size and shape to modern Homo sapiens. These have been dated at 80,000 years. A similar skull found in a cave at Qafzeh, Israel has been dated at 9l,000 years. Having the same size brain capacity, of course, does not necessarily mean they had our same intelligence, although they were capable of making beautiful stone tools.
We jump now to the Chevaux cave in France, where wall paintings of animals extant in Europe at that time are beautifully depicted and have been dated at over 30,000 years. l5,000 years later in the caves at Le Portel and Lascaux in France, our ancestors were making magnificent polychrome paintings of animals. Their stone tools at that time and for the previous 5,000 years are comparable in technique and beauty to any made by Native Americans in the past few hundred years.
Can anyone doubt that these Cro-Magnons could have learned to read and write, to philosophise, to do math at a high level, to learn chemistry and physics if magically brought into our culture of today? (Let's leave out some fundamentalists who still don't believe in evolution.)
We find that cuneiform writing began about 5,000 years ago and quickly evolved. By 2,500 years ago, the Greeks were producing masterpieces of plays, literature, art, architecture, and they were doing some wonderful things in mathematics and elementary observational science. The Romans carried on these traditions until their fall. Christianity came in and destroyed as much as it could of this great heritage in western Europe including the great library in Alexandria.
Thus began the "Dark Ages" in Europe. The gradual dissemination of knowledge, other than ecumenical literature (which wasn't much faster!) was extremely slow. So, in the mid fourteen hundreds along comes Gutenberg with his printing press and its movable type. Of course, almost the first thing he did was print a bible or two and they sold like hot cakes. True, fixed or non movable type had been around for a short time, but the process wasn't much faster than printing books by hand and was very costly, so, the rapid dissemination of knowledge through printed books began with Gutenberg.
While it is true that the Dark Ages began to end about the year l,000, real progress wasn't made until the Renaissance and, particularly, with the rapid dissemination of knowledge via Gutenberg-type presses. As books were published, people became inspired to learn to read. Reading led to thinking about what had been read and to further publications and to communications between people. The first world wide web had been started, Anyone with a grain of sense can see what this has led to!
So, John, after my consideration outlined above, I still think the Gutenberg press with movable type is the greatest invention of the past 2,000 years or, perhaps of the last 5,000 years after cuneiform writing was invented!