The DCU is has a long legacy of world-ending events, each one typically being even bigger than the one before. Over the course of years the DCU has almost ended many times, so many in fact that it almost is expected from fans that every couple of years they get to see another huge crisis.
Each major event tends to set up a new status quo for the DCU and helps to usher in a new age for DC. While the heroes always save the day, it's always a fun journey for fans to follow along with.
10 Crisis On Infinite Earths
When the Anti-Monitor came to Earth it spelled the end of the universe for all the beloved heroes of the DCU. This battle even cost the lives of Supergirl and Barry Allen and it did to some degree destroy a good portion of the DCU. It reshaped the multiverse by collapsing it all into one singular planet instead of multiple planets. Crisis on Infinite Earths is considered one of the best comic events of all time and it certainly lives up to it.
9 Darkseid War
Once again the Anti-Monitor is involved in the impending doom of the universe. Darkseid and the Anti-Monitor bring their showdown for the Anti-Life Equation to Earth and cause untold destruction. As they battle the equation is released into the heroes causing them all to be their worse selves. If Grail and Darkseid hadn't been stopped on Earth, the whole multiverse would have been doomed.
8 Dark Nights: Death Metal
The most recent DC event was a big one that set up the next stage for DC, Infinite Frontier. The Batman Who Laughs rebuilds the world in his image and fights the heroes for the fate of the entire omniverse. As the hands of the creators reach out to finish reshaping the world, Wonder Woman stops them and agrees to take her place as a defender of the new multiverse.
7 Final Crisis
Darkseid is one of the biggest threats that face the heroes of the DCU. He is always just around the corner waiting to launch his next attack on the galaxy. Final Crisis was one of the last big crisis before Flashpoint, Darkseid and his minions secretly invade Earth and systematically take out Earth's best defenses. Darkseid even kills Batman with his omega beams, but the real threat is Mandrakk, The Dark Monitor. Mandrakk waits in the shadows for everyone else to fall so he can consume what is left of the universe.
6 Zero Hour
Zero Hour is about Parallax aka Hal Jordan taking out his rage and grief on an unsuspecting world. He wants to remake the whole world in his own unique vision. Jordan is doing this in order to erase all the events that lead him to his path of insanity.
Hal Jordan goes up against all his former colleagues, and as usual, the villain fails. Even though Hal eventually came back to the light after seeking redemption, the legacy of Parallax hung over him for many years.
5 Flashpoint
One of DC's most famous and popular events is Flashpoint. This saw Barry Allen create an alternate reality when he tried to save his mother from being murdered. In this reality, the world was in the midst of World War III and Wonder Woman and Aquaman were tyrants. Bruce Wayne was dead and his father was Batman. The whole timeline was a mess and was saved by Barry once again sacrificing his mother and his happiness.
4 Infinite Crisis
The opposite of Crisis on Infinite Earths, this new crisis of Infinite Crisis was all about expanding the multiverse again, or so the heroes were lead to believe. Superboy-Prime and Alexander Luthor of Earth-2 wanted to rewrite Earth-Prime so it is like their homeworld of Earth-2 as well as restore multiple Earths to their rightful place. This crisis marks a turn for Superboy-Prime and would have changed everything for these fan-favorite heroes.
3 Dark Nights Metal
When the Dark Multiverse invaded Earth it looked like the end of times for the heroes. The dark bat-god Barbatos wanted to drag everything down into the darkness to suffer as he had. Batman had to fight nightmare versions of himself at every turn, each one viler than the last. Even Hawkman had been imprisoned down in the dark. Barbatos was almost successful but the heroes rallied and defeated the dark god.
2 Year Of The Villain
The Year of the Villain was a multi-stage plan that showed how Lex Luthor and the Legion of Doom helped Perpetua rise to power and start her path of destruction across the multiverse. It started by Lex gifting the villains increased abilities to allow them to better cause chaos and destruction, and it climaxed with the world choosing Doom over hope and happiness, which allowed Perpetua her final foothold in the universe.
1 Invasion!
When Earth became too powerful, the aliens called The Dominators came to take out that threat. They created a gene bomb that would wipe out the metahumans on Earth, therefore eliminating any threat there could be to their invasion. The Dominators were successful in dropping the bomb, but the heroes were still able to send them packing in the end.