This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Freddy vs. Jason
The flashback sequence at the beginning of the film. That along with Freddy's narrating shows every terrible thing Freddy's ever done to the kids of Springwood. And he's proud of it.
And Freddy screaming "I CAN'T COME BACK IF NOBODY REMEMBERS ME!"
Lori's "Please wake me up!".
And a little later in her dream where Freddy slides his finger blade up her thighs while she is sleeping, and pulling her skirt up, revealing her bare legs as she whimpers and writhes on the floor.
Mark's nightmare is probably the scariest scene in the film. He quickly realizes exactly what's going to happen and that there is absolutely nothing he can do about it.
"Somebody please wake me up! Please!"
Freddy Krueger's past is laid out in a prologue that features an offscreen murder of a little girl — who's later seen in a dream sequence with her eyes cut out. Say what you want about Freddy's Villain Decay, but he got his reputation for a reason.
Basically, that was the scene that instantly reversed two-decades of Villain Decay for Freddy: even with the stupid gags, even with the lame jokes, he kills children for fun.
There's also the way he slowly licks her photo before pasting it in a book filled with newspaper articles about missing kids in Springwood and photos of all the kids he's killed. It's quietly terrifying and leaves you feeling fouled just by seeing it.
Freddy's torment of a young Jason after their Dream World fight is horrific — especially considering Jason is a little boy and Freddy is a child murderer.
If that's not bad enough, in one of the unused scripts for the film Freddy was to be a counselor at Camp Crystal Lake and molest him.
The opening scene of Jason's nightmare - assuming it's accurate to his history and not Freddy making it seem even worse - makes his origin-story even crueler than it had been, revealing that he hadn't just been out swimming when he drowned. A pack of bullying young campers chased him into the lake, taunting and throwing things at him, then stood jeering on the shore to watch him die. And presumably never owned up to it, given Pamela Voorhees' sincere belief that it was solely the counselors' fault.
Kia lampshades the nightmare of having to deal with both Freddy (who attacks in dreams) and Jason (who attacks in reality) with one line: "We're not safe, awake or asleep!"
Jason's dream at the beginning of the movie. It's a typical scenario that ends with Jason killing a lusty half-naked camp counselor in the woods at night. Then he starts to hear his mother's voice calling him. Then the girl he just killed comes back to life, morphing into his other victims, all of them saying they deserved to be punished. After this, Pamela Voorhees appears. It's a very eerie look into Jason's psyche, even if it does turn out to be part of Freddy's plan.
Pamela: Jason. My special, special boy. Do you know what your gift is? No matter what they do to you, you cannot die. You can never die...
There's something mildly disturbing in seeing Jason come back to life. You get a close up of his open chest cavity, with discolored bones and decaying organs that have vines growing through them. And that excuse for a heart starts pumping.
That and when Jason is woken up, his kid self vanishes, leaving Freddy so pissed, he looks up at Lori before lunging out of the water, as everything turns red, now looking more like a burnt demon version of himself, making it perfectly clear that now, he's not gonna hold back with her.
Freddy lures a drunken, sleepy Gibb away from the rave by taking on the form of her dead boyfriend. This being Freddy, he doesn't make him at all appealing and in fact just looks like a living, clothed version of the broken, mutilated corpse Jason left behind. It's creepy watching him stagger off into the cornfield and threatening her if she doesn't follow.
Gibb is freakishly unlucky. First she's traumatized by the sight of her brutally murdered boyfriend (who was already verbally abusive), then she's lured into a dream by Freddy who slowly hunts and terrorizes her. Immediately after passing out in a cornfield, a man starts molesting her unconscious body. Before Freddy can kill her and the man can rape her, though, Jason impales both her and the rapist. Poor girl could not catch a break.
Just imagine, you're at a rave, probably a little drunk or stoned (and definitely not taking the recent murders seriously)... then Jason comes walking out of the corn field, engulfed in flames, and starts slaughtering everyone in sight as the fire starts to spread. Hope you have the car keys, otherwise you're in for a very ill-fated run for your life.