To date, what do you think is the greatest invention or discovery and why?

It's pretty damn hard to pick just one thing, so my best-of list

There's really basic foundational things like the wheel, cutting tools, fire (if we want to count it as an invention,) string/rope/cordage, writing, clothing, cooking, agriculture, metalworking, etc. the sort of things that are absolutely basic building blocks of civilization.

Moving a few milenia up, and in no particular order,

the Haber Process to synthesize ammonia, which allowed for the creation of synthetic fertilizers. If you've eaten any commercially grown food in the last century, you probably owe it to the Haber Process.

Antibiotics are another big one, as are vaccines.

Vaucason's lathe arguably laid the foundation for a whole lot of fabrication techniques that led to the industrial revolution

Refrigeration

Steam engines and later internal combustion engines

Clocks

Compasses

Printing press

The telephone

Airplanes

Computers and the internet

Cameras