This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Franken Fran
This manga is not for the faint of heart. Or for those with a particularly strong religious conviction. (Not that it portrays religions badly in most cases, just the content of the work.) Or those who don't want to see human entrails rendered in loving detail.
The result of Fran's operations, most of the time resulting in And I Must Scream for the patient, as you can see below.
The installment where a lovestruck student convinces Fran to revive the girl he's sweet on after she's pulped by a truck. The result of Fran's work: Her head is on an enormous caterpillar body. The guy helps her adjust to the new body, and when she later forms a chrysalis and emerges seemingly restored to normal, they decide to find a hotel room. The final scene shows her transformed to a hideous half-human/half-insect form and devouring his guts while he screams "It hurts! Hurts!" The couple apparently had sex right before this, as the girl is shown as having a baby in the chapter Egg Parturition, or chapter 29.
In general, Fran can look pretty damn scary when she's about to perform an operation, especially in the early chapters. In chapter 5, as she's about to give the spy her comeuppance, she almost looks like a villain if you take it out of context.
Chapter 7 is about the relationship between an insane schoolboy and a girl. It's made obvious from the start that he often has violent mood swings that cause him to lash out at people, and it's implied he stays violent with his girlfriend even after Fran revives her. Worse, since the girl is essentially just a head without her memories or any way to defend herself to run away, she can't do a thing about it. At the end of the chapter, the boyfriend finds inscriptions under the bed. They were written by the girl, thus proving she didn't lose her memory. Worse for him yet, after he lashed out at Fran earlier, the latter gave the girlfriend a new body. A monstrous body. The chapter ends with karma hitting the boy like a steamroller, as the girl urges him to open the door,so she can finally "hold him in her arms". The girl isn't even his girlfriend. She's just a random girl that he attacked; he doesn't even know her name. He killed her because she didn't hug him back when he assaulted her.
Then there's Gavrill who first appears in Chapter 40. Never mind the fact that she's a cruel and sadistic killer. Never mind the fact she was first introduced in the manga as she was eating someone's remains. Never mind the fact that she flat out tears Veronica apart. It's what she can become that puts her into this territory. And she always appears to be very hungry. Very, very, hungry.
The absolute worst is Chapter 21 with the mascot. A boy is accidentally used as body parts by Fran when she thinks he committed suicide (he was really taking a nap) and is put into a mascot costume until she can give him a body. He meets a little girl who is being sexually abused by her stepfather. A panel actually shows her being raped. After the boy decides to kidnap the girl to try to keep her safe, he's hunted down and just finally manages to attack her stepfather. We see him waking up in the stepfather's body, Fran having given him it as a replacement, and he's ready to be a better father for the girl. Happy ending, right? WRONG. Fran ends up putting the pedophile stepdad into the mascot costume and remarks about how happy he was now that he can be around children all the time. Fran even takes him to the amusement park the mascot is from so he can live out his new 'wonderful life'. Easily one of the darkest and creepiest endings in the series.