I agree that Science is the most important human development is the last 2000 years, but it doesn't quite qualify as an invention. I therefore propose the clock as the greatest invention, since it is an instrument that enables Science in both a practice and temperament.
The clock is the embodiment of objectivity. It converted time from a personal experience into a reality independent of perception. It gave us a framework in which the laws of nature could be observed and quantified. The mechanism of the clock gave us a metaphor for self-governed operation of natural law. (The computer, with its mechanistic playing out of predetermined rules, is the direct descendant of the clock.) Once we were able to imagine the solar system as a clockwork automaton, the generalization to other aspects of nature was almost inevitable, and the process of Science began.