If we talking most widely used, not the most useful then it probably be one of the three.
Internet. Currently there is more that 30 billion connections to the net, that is average of 4 connections per person. (1 at work, 1 at home, smart electrical meter, smartphone, car, alarms, video cameras and so on) Most people currently do not use fire and wheels ( If do not count the mouse wheel) so much, but everybody are in the Internet. Even you reading this answer.
Cotton. For a very few exceptions every person in the world has something made from cotton or a cotton blend on their person right now. If you don’t, then you probably used some paper recently that is also made from recycled cotton.
Toilet paper. It is the most selling and used product in the world. We eat different food, live in different houses and wear different clothes, but we all buy and use toilet paper.