Antibiotics, vaccines, clean water and sanitation. If you think back several generations, say your great grand-parents, the diseases that were deadly then (TB, smallpox, scarlet fever) are almost gone.
My choice is between the wheel and the Internet. The wheel enabled people to move around and travel large distances. In the 20th century, I think the Internet has been highly influential. It has revolutionised the way we communicate and exchange information.