This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Eureka Seven
Renton's demonic-level rampage in Episode 20. After being sent into an Angst Coma brought on by hatred towards Holland, feelings for Eureka, and hate towards his immaturity, Renton has the mother-of-all-freakouts as enemy mechs are swarming him. He goes into a Foe-Tossing Charge, maiming, bisecting, gutting, slicing, and smashing every single machine roaring toward him. Worse, Nirvash is soon sporting RED EYES FROM HELL.
This is only the half of it. It got a HELL of a lot worse when his stress reached it peak. He imagined a shadow coming at him with his face, but twisted into a psycho Slasher Smile, and letting off a maniacal Evil Laugh. Renton suitably unleashes a bloodcurdling scream and loses it,◊ dismembering the very last mech that attacked him, which had already been thrashed into submission. It's been dismantled to the point you can see the cockpit of the man piloting it. However... Renton, with a final blood-curdling scream, throttles Nirvash's foot right down on the human, crushing him like a cockroach. When Renton has calmed down, he raises Nirvash's leg... and you can see a SEVERED HUMAN ARM sticking to its foot slough off and splatter to the ground, with GALLONS OF BLOOD spilling off the edge. Worst of all, it has a wedding ring on the finger. As soon as Renton spots this, his eyes snap open with genuine horror. You hear a few staggered heartbeats... then the poor kid throws up from shock and repulsion. The next episode, he's stuck in a Heroic BSOD, and finally cries his eyes out for several agonized minutes.
The sequence where Dominic watches how they intend to make a replacement for Anemone, which fails horribly.
It's worth noting that that scene earned that episode (and only that episode) a TV-MA rating in the US. Can't imagine why...
Anemone, from her disturbing ways of eating jam to being obsessively dependent on drugs, one can't help but be horrified watching her in the series.
In the episode "Paper Moon Shine", we're treated to a flashback from Eureka's point of view, detailing the destruction she caused as an SOF member. At one point her Nirvash is disabled by a group of civilians, and when they think they've managed to stop her she gets out of the cockpit and mows them all down with a machine gun. Not only is it disturbing to see Eureka be so utterly emotionless, but when she is inspecting the bodies she kicks a couple over. Underneath she finds Maeter, Maurice and Linck, the children she herself orphaned, looking utterly traumatized, and clinging to what was most likely their parents. It's worth noting that this is what prompted her Heel–Face Turn.
Holland's nightmare at the start of the episode "Higher than the Sun" is nothing short of pure Nightmare Fuel itself, as Holland is forced to see the death and carnage he caused as a member of the SOF.
In the episode "Opposite View" Renton, Eureka and Anemone begin having a synchronized dream of sorts. It starts of with Renton apologizing to Eureka and confessing his feelings to her as they sit in a vast, white room. When Renton asks her if she is ignoring him, they look up and see the face of theEnd. The room goes black, and blood pours from the ceiling before theEnd's mask falls off, revealing Anemone's face displaying a frightening Slasher Smile. When she tries to attack Eureka, Renton suddenly transforms into the Nirvash and fights Anemone off, pushing her back into the ceiling as she screams and wails. To cap off all of the Mind Screw, Eureka tells Renton that she heard everything he told her in the dream.
The episode "Into the Nature" also serves as our introduction to Anemone and the type theEnd. When Renton and Eureka try to enter the Coralion with the Nirvash, theEnd swoops down and attacks them. It finally catches the Nirvash in its claws, allowing Renton to see that the two mechs look shockingly alike. Anemone then unleashes theEnd's Mind RapeWave Motion Gun, causing Renton and Eureka to scream in pain and beg her to stop. Anemone only makes things worse as she squeals psychotically.
Anemone: MELT AWAY! MELT AWAY! YOUR BRAIN'S GONNA MELT AWAY!!
It goes From Bad to Worse in the next episode, "Acperience 1", which begins with Renton, still getting thoroughly Mind Raped by Anemone, screaming and begging for his sister to help him. Eureka then sees him grab the Amita Drive, and as she joins him they activate the Seven Swell effect and knock Anemone's attack back at her. The three pilots then enter the Coralian, spending the rest of the episode wandering around in a dream world filled with more Surreal Horror and Nightmare Fuel than you could count.
Renton: Help me! Help me! HELP ME! HELP ME! HELP ME, SIS! HELP ME!!
The title character was a bit of nightmare fuel by herself before the series. Eureka's actions, in the SOF were revealed in a flashback to the suppression of the Vodarac in Cuidades del Cielo, which basically amounted to genocide. She was using the Nirvash to gun down a group of Vodarac followers, when the Vodarac responded by firing back a grenade launcher, disabling the Nirvash. Thinking that it was over the Vodarac begin to celebrate this small victory only to see Eureka climb out her cockpit and and gun them down with an assault rifle.