Well, that depends. Are we talking militarily or for regular civil life?
Examples are the Roman reverse overshot water-wheel, and the Greek made first steam engine in the 1st Century AD, called the Aeolipile.
For a medieval invention, you could always go Trebuchet. Johannes Gutenberg's printing press was also a major innovation as prior all books had to be hand written from first page to last, and it made it much cheaper and laborsome to make books, which lead to the Reformation as the Bible got translated and kicked Latin as the lingua franca of Europe
Less famous, and commonly used is a hand crank.
Hope that helps : )
