3. The Yuuzhan Vong War
The war against technology hating aliens the Yuuzhan Vong in the New Jedi Order series caused a total of 345 trillion deaths, making even more catastrophic than the Galactic Civil War. The were eventually defeated, but the galaxy would take decades to fully recover from the sheer amount of death and destruction. In the current canon, the First Order seem to be doing a perfectly fine job of destroying the galaxy by themselves, but who knows? Maybe the Resistance will finally defeat them for good, and the New Republic will finally restore peace to the galaxy. Disney will need a new threat to keep the cycle of movies going, and maybe they'll look to the EU and choose the Vong to be the next major villains for new films down the line - they are fully formed. After all, Fans might eventually grow tired of seeing armies of stormtroopers led by dark force users again and again and crave an all new threat to the galaxy.