The bicycle.

Hear me out:
Before the invention of the bicycle, the vast majority of the population had no means of personal transport other than their feet, and anything further away than the nearest market might as well have been in China, cause neither a farmer nor a worker with a family can just take more than a day off.
This meant that almost no one ever travelled further than 30km from their home.
With the bicycle, the world that most of humanity got to experience became 20x bigger.
People met other people further away, experienced new ideas, could travel outside of the immediate influence of their landlord or master, could marry someone who isn't a cousin...

No other invention ever before opened up the world of the average person quite like this one.

The bicycle created demand to build a dense network of smooth roads even in the countryside, brought workers to factories, and gave women more freedom. It was one of the main factors that pushed the industrial revolution.