This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Kingdom Come
"Armageddon has arrived."
If the gangs of supers fighting it out in total disregard to the lives and welfare of civilian bystanders doesn't do it to you, then consider the descriptions for Superpower Meltdown, Beware the Superman, and Unstoppable Rage, and put yourself in the shoes of one of the non-supers. Oh Crap! doesn't begin to describe that puckering sensation you should be feeling.
Billy Batson's torture and brainwashing at the hands of Luthor. "Mistu... mistu... Mr. Mind."
The end when we're shown the effects of the nuclear bomb? In a full page, even? Dear God...
The Fridge Horror over the fact that Lois was killed with Joker gas.
This was subverted in the follow up story in Justice Society of America. Superman revealed to the Lois of New Earth that his Lois had actually died from a head wound. She survived the Joker gas via a gas mask she kept in her desk and tried to take the Joker on with a fire extinguisher, only for him to bash her head in with a paperweight. However, she survived long enough for her to die in Superman's arms - which how it was described in the Novelization.
In-Universe Example from the Novelization: when the Joker is arrested after the Daily Planet incident, he's his usual giggling, mad self. Then Magog shows up, and the Joker is truly terrified by what's about to happen. Then Magog fries the Joker's midsection into vapor.