Television followed by the Internet, then lightbulbs, then motion pictures.
1. Television, because it exposes us to more culture then we think it does. Despite the hundreds of hours of 'Joe Schmo', 'The Bachelor' and all the other crap, today's generation knows more about other countries because of TV then the kids 100 years ago.
2. The Internet, because the speech becomes two way and we can get stories from places that were silent and learn things that have been locked in libraries from the convenience of our homes (or offices).
3. Lightbulbs because life doesn't stop at sundown anymore. We can save lives in surgery, read books at bed time, and secure our houses from the cowards that creep in darkness.
4. Motion pictures because without them, pornography would be boring.