Over the last 100 years, there have been quite a few ideas and inventions. Computers, WWW, landing on the moon, and TV, etc. Some of them were completely changed our everyday life.
I my opinion, the number one event was happened in 1923 when Edwin Hubble found that the nebula is another galaxy. He concluded that Andromeda must be at least 10 times farther away than the farthest stars in the Milky Way. This discovery meant that the other spirals were probably also galaxies much farther away.
Hubble's work changed forever our view of the universe. Milky Way galaxy was NOT the entire universe!
While the landing on the moon made us proud of achievement, the discovery of another galaxy other than Milky Way galaxy made us deeply humbled.
Though there are billions of other galaxies, just to get to the nearest galaxy (Andromeda), we need 2.5 million years with light speed.
When we are watching the Andromeda in the night sky, the light we see was emitted when there were dinosaurs on the earth. It's too much.
Copernicus and Galileo humiliated us 500 years ago, the discovery of Andromeda not belongs to our galaxy made us think ourselves again.
NOTE: Above information has been taken from wikipedia and/or official websites of topics.