I have always wanted to have created this thread but for some reason never have.
As it would be IMO to point out one single greatest invention, I have gone for people list more then one.
For me the basics at the beginning are, with the invention of tools to cook, hunt, build, all to survive and to make clothing as well. Because with out them you couldn't survive without them (or at least struggle), food, water, shelter, and clothing are the most important things you need to survive on, no matter what. Eventually these tools that were made for hunting to make weapons when other countries went to war which to did lead to wiping out whole populations of people. I guess this is how poeple from 1500 years and beyond survived.
But for me it is the invention of the printing press, which has undoubtably by experts as the world's greatst invention by Johann Gutenberg in the German town of Mainz in the 15th century. This progressed onto publishing 200 bibles that were sold in Europe, but actually he never made any money out of it, with the money he got from the sales of the bible went to paying off his own debts.