Hmm... That's honestly a tough cookie. I would say gunpowder though, because it helped them strengthen their militia, and that is a very important aspect in any sort of country or empire that wants to be respected. It also helped them develop deadlier weapons for their naval forces, which the Chinese empire was very advanced in-- think of Britain in the 1800s but long before Britain was even an idea. Also, as it was such a deadly instrument, many other nations wanted to get hold of it, and China could get wealthy from trading it.
