What are the most important inventions since flint tools and why?

Well... Rival Holland had coffee, and so shaped their world around Coffee; went to Java for their Java, we wanted tea; China needed nothing from us, so we got them addicted to Opium. (True story), but we wanted their tea so bad we invented Corporate Espionage to do it when we sent Robert Fortune to China to steal it for us.

A coffee house in Amsterdam, NL doesn't specialise in Coffee, by the way, but they had colonies where the coffee was.