This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Barefoot Gen
Witnessing anything past this image will ruin your entire week.
The depiction of the bomb's actual explosion and all the living zombies is terrible enough, but the most personal instance has to be when Gen runs back to his home to find his entire family except his mother trapped under their burning home. Neither of them can lift the debris off of them. After his father convinces Gen and Kimie to get away from the fire, the two watch as the other three burn to death, his sister and brother pleading for help the entire time. It's no wonder Kimie goes temporarily insane after watching that.
That said, the scene of people melting as the bomb strikes—particularly the woman holding a baby—is downright horrifying, and only made worse by the fact that it goes on, and on, and on.
Just as bad, if not worse, is the scene of the dog trying to flee in terror. Most of the people shown have so little time to process what's happening that they can barely react, if at all. The dog, however, can tell that there is danger coming and tries in vain to run for its life. Poor doggy...
Well, the whole film, as it is meant to show that War Is Hell, after all. But especially the living "zombies", and the fact that they were not zombies, but people who were badly injured (and most likely disoriented). The skin melting off and eyes melting out of sockets is beyond horrifying.
Other horrific details include corpses bloating up in the hot sun and sometimes rupturing, as well as the guy who shambles about dragging his intestines behind him.
The idea that your whole life could be turned upside-down and darn near everything you know destroyed in seconds...all because of some enemy nation's Applied Phlebotinum.
The simple fact that this actually happened. And that someone survived the horror. To look upon most of them—and in retrospect, even Gen—maybe it would have been more merciful for them to have died right then and there.
An even scarier one, since its more personal. Gen sees his father, younger brother and older sister trapped underneath the rubble of their home- then he hears them scream in agony as the flames grow closer, his father encouraging his increasingly desperate mother to leave them to die. As if things weren't bad enough, we're treated to a LOVELY picture of three out of five members of Gen's family burning alive right in front of Gen and his mother.
In addition to that, although it is mercifully brief, Gen's mother Kimie's expression during her momentary insanity is downright terrifying. This, her madness mantra and insane laughter underscore how utterly alone Gen is at that moment, with his mother incapable of taking care of herself, let alone of Gen, and with the flames closing in on them.
More psychological than physical, but the ostracism and harassment that Gen's family suffers in the manga as a result of his father finally openly saying what he thinks about the war to block chairman Samejima and the whole neighborhood during spear drill (remember, this is in April 1945 meaning he's kept it bottled in throughout the war) can be uncomfortable reading in certain parts, like when Daikichi is arrested by the police, taken away and tortured.
The suspense that begins once Gen's hair starts falling out after the bomb detonates. Most likely, this was caused in strong part by atomic radiation in the area (although other factors, such as immense stress, could have arguably played a role). What other effects, both short-term and long-term, will Gen suffer from thanks to his exposure to radiation? Reading the story for the first time, one would half-expect it to end with a young boy dying a slow and painful death.