This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Paranoia Agent
Paranoia Agent is a series from Satoshi Kon. Its premise is a kid on rollerblades skating up to knock you out if life is getting a little bit much for you.
A grade school kid being driven to assault and then insanity by his classmates suspecting him of being Shounen Bat/Li'l Slugger. As he turns insane, we are treated to a view of his new outlook on life. A technicolour nightmare of floating people with distorted faces that all suspect him of beng Li'l Slugger, and the only clear image in it is Ichi himself and the actual Li'l Slugger - gliding towards him with his bat held high, all while Ichi's theme "Jouken Douji" or "Condition Boy" plays.
How fast his "friends" and most of "supporters" turn against him is nightmarish by itself. The kid did nothing (except resemble someone), and, over night, all of the people who cheered him at school begin spreading rumors, ostracizing him, and leaving threatening notes in his locker. He didn't even know why this happened at first.
A woman with disassociative identity disorder waking up to recorded messages from her other personality on her cellphone, that become gradually more threatening.
The "Mellow Maromi" episode.
It's focusing around a production manager who is the only surviving member of his team at an animation studio, having killed them one by one. He drifts off several times into increasingly nightmarish flashbacks - only to wake up back in his car, plagued with the suspicion that Shounen Bat is right behind him. Of course, he is. At one point during a car scene, Shounen Bat can be seen skating leisurely along in the rear view mirror - with no fanfare calling his appearance whatsoever.
The Offscreen Teleportation just gets ridiculous here. The manager drives the car at over 120 mph? Lil' Slugger gets ahead of him and knocks on the window. Rams Lil' Slugger away? He just appears in the goddamn backseat. Yes, even if you're in a speeding car, you can't escape him.
The producer. He seems like a stern yet reasonable fellow at first but then the attacks happen and you can see the stress building up. By the time the animation is finished, the editor is slumped dead on the computer console. But the producer just grabs the tape from his hand, happy that it's finally done. Oooookay.
That goddamn Otaku and his dolls, even if they were helping "Radar Man".
"Call me Daddy".
A teenaged girl finding the hidden camera her father has been using to take pictures of her undressing.
When Maniwa starts losing his grip on reality throughout the 7th episode, including thoughts tinged with static and creepy hallucinations of the old man and Lil' Slugger.
Maniwa and Ikari seeing who they think is Kozuka in the shadows outside his cell. And then they realize who it really is when they see the bat dripping with blood and inhuman golden eyes.
Some of the stories made up by the Gossipy Hens are pretty freaky to watch, particularly the stressed-out student who suddenly vomits all of his knowledge onto the floor in a wave of black text. Then, at the end of the episode, the newbie gossiper comes home to find Lil' Slugger has nearly killed her husband, and she just starts shaking him while demanding in a psychotic tone of voice that he give her all the details.
After he nearly succumbs to Talking the Monster to Death, Lil' Slugger turns into a hulking monster with jagged teeth and Blank White Eyes, and begins chasing Sagi down. At one point his arm is attacked, showing the bat to literally be a part of him, and then it grows back.
At first glance, the end credits, which show everyone sleeping with a faint smile around a giant Maromi in a empty grass field, seems kind of blissful. That is, until you realize not a single person there is breathing.
Watch Puella Magi Madoka Magica, then go back and watch the final two episodes of this series. Good luck not being extra creeped out by the 2D world. Its disjointed animation and the Despair Event Horizon trigger are very much like a Witch's Labryinth. Episode 13 becomes oddly similar to the battles between Homura and Walpurgis Night.
The ending: After Sagi takes responsibility for the death of her puppy Maromi (which lead to the creation of her creation of Maromi) and stops the reign of terror of the Black goo, the show cuts to two years later with Tokyo recovering and some of the characters recovering now (Ikari still working as a security guard and such), and yet we see Maniwa who's become similar to The Old Man, solving a complex math question. And then he gasps.
Add more that the show fully closes with another "Prophetic Vision" with Maniwa who's fully taken the place of the Old Man and in a sense, invites the viewers to watch the show again with a bigger understanding now. All seems normal and good until you realize the entire show is basically a Vicious Cycle.