Birth control (or if you need it to be more specific, the pill)
Why? Because freedom from constant childbearing means that women can become meme-spreaders like men — working for their memes rather than their genes. This then means a change in the kinds of memes that propagate effectively, including all the memes of other inventions as well as the meme-spreading media, myths, science and the arts. In other words, it is important because it changes the whole of culture. Few single inventions have this effect on the whole meme pool.