What is the biggest, most famous (In universe) event?

As was Sion and Nihilus' shadow war against the jedi.

The fall of the Empire
The fall of the Republic and Jedi Order

The Movies are the greatest events because George made movies of those things, not the EU. Therefore, TPM-ROTJ is what is the most important to the Star Wars Universe.

Order 66 takes it.

no, Destruction of the empire.

1.) Discovery of the Hyperdrive.
2.) Founding of the Republic and Jedi.
3.) Schism.
4.) Great Hyperspace War (revealed to the galaxy the Sith)
5.) Battle of Ruusan. (lulled the galaxy into a false confidence)
6.) Order 66
7.) Declaration of New Order
8.) Destruction of Death Stars.
9.) Final death of Palpatine.
10.) Yuuzhan Vong invasion.

Pretty much the most important, famous, and biggest is the very beginning, since nothing would have happened like it did if the beginning had not happened like IT did.