Earlier contributors have already staked out the intellectual high road of mental constructs like scientific method and the calculus, so I'll retreat to the most prosaic, literal reading of your question: What is the one device invented by one person at one moment during the past 2,000 years that has had the most influence to date?
I'd be a traitor to my inky profession if I didn't at least echo the nominations for Johann Gutenberg's movable-type printing press. But in the spirit of the game, let me throw support behind something else: Alessandro Volta's electric battery of 1800.
Static electricity was known since at least the time of the Greeks, but study of it had largely stalled. When Pieter van Musschenbroek built and discharged the first Leiden jar in 1745, nearly killing himself in the process, he also jolted the study of electricity back to life. But it was Volta's invention of a steady source of current, inspired by the electrochemical observations of Galvani, that revolutionized technology and physics. Without it, Oersted could not have proved that electricity and magnetism were different faces of the same force, electromagnetism. Electrochemistry itself offered clues to the underlying electrical nature of all matter. And of course, Volta's battery was the forerunner of all the electrical devices that have transformed the world over the past two centuries.
What I find so appealing about Volta's creation is that it had immense practical significance but also opened to us a world of physical phenomena that in themselves changed our understanding of the universe. Yet it was not a bolt-from-the blue inspiration; it pulled together other threads of discoveries by Volta's contemporaries. There's a lesson about greatness in there somewhere.