This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Paranormal Activity
The First Movie
The final shot of the possessed Katie sniffing at her boyfriend's corpse before smiling at the camera then twisting her face and letting forth an inhuman shriek as she launches herself at it.
Katie's screams in that scene.
The truly horrible moment is three quarters of the way through when, after it being well established that all hauntings happen at night, a photo cracks. At noon. This is closely followed by Katie's realization when the demon's footprints appear in the powder. Why on earth would they appear anyway? Because the demon wants them to.
"How come my face is scratched and yours isn't?"
In one alternate ending Katie gets out of bed and stares at Micah for 3 hours straight. Then walks downstairs and starts screaming. Micah runs down, and he screams, and suddenly you hear 3 people screaming. And then Kate comes back up with a knife and slits her throat. Disturbing is putting it mildly.
Another had Kate staring at Micah, going downstairs, screaming, then killing Micah but instead of slitting her throat when she comes upstairs, she just sits there rocking her body for days, the fast forwarding of her rocking only interrupted to get the viewer a chance to hear a phone call (answered by the machine) from her friend. She just keeps rocking. It ends with the police coming to investigate. Kate finally comes to for a bit and starts walking towards them, still with the knife in her hand, and they shoot her.
On second watch, the "Cop" alternate ending becomes even more terrifying. As the cops enter the house and come up the stairs, the lights in the hall flicker on their own accord. The demon? HE'S CONTROLLING THE ENTIRE HOUSE AS WELL AS KATIE. And then, you see Katie get up confused and disoriented... the demon left her body. And as the cops draw their guns on Katie as she comes with the knife, watch the door in the background. The cops didn't shoot Katie because she was threatening them. The demon SLAMS the bathroom door, and the resulting sound gets a cop to pull the trigger.
The very real portrayal of their relationship breakdown. Whether the demon was real or not, it was clear that Katie was terrified. She begged Micah to stop filming, she begged him not to play its game. What did he do? Completely ignore her and make things worse.
Scariest bit in Paranormal Activity? The bit with the flour on the floor and demon tracks. Funny thing, really, considering most demons are depicted with their feet backwards. As in, it's in the room with them.
Micah is hurled into the camera by the demon. It wasn't really the act itself as much as it was the roar the demon made while doing so.
That last second at the end before the jump scare, when she's looking into the camera and smiling horribly. It can feel as if she is staring right at you, the viewer.
"I think we'll be okay now."
Towards the end of the film, when Micah is all set to leave for a hotel, Katie suddenly claims that she'd rather stay home. Paying close attention, you can tell she's not speaking normally. In fact, she's speaking in much the same way she did on the swing earlier in the film, like someone else is trying to do a convincing impression of a human. If it wasn't obvious already, the slight double-voice you can hear as she says, "I think we'll be okay now," followed by a small smile while staring right into the camera, should make it very clear that she's possessed. It's subtle, yet still noticeable to the attentive viewer, and so it's absolutely horrifying and makes you realize that whatever hope you may have been holding onto for the characters is best abandoned.
Katie being dragged out of bed and down the hall by her feet is terrifying to watch, especially since the demon had largely stuck to smaller, much less harmful scares up until that point.
An early jump scare features Katie and Micah hearing a loud BANG from downstairs... but what really makes it terrifying is the demonic howl right before the bang.
On a couple of occasions, when Katie is possessed, she gets out of bed, and stands in one spot. Staring at Micah. For hours. It's a lot creepier than it sounds.
On a more meta level, some of the real terror of the film becomes apparent if the supernatural elements are ignored: now it's a film about home invasion, about how you aren't necessarily safe even in your own bedroom, and how being asleep and oblivious doesn't make you any safer to what might be going on during the night.
The Second Movie
Kristi sitting in her kitchen and suddenly looking up and behind her is tense, because you know something's about to happen. Most viewers aren't going to expect every cabinet and drawer in the kitchen to burst open with a loud bang. It startling beyond belief.
Kristi trying to cope with a Stepford Smiler attitude immediately after doesn't help.
Kristi's possession. Just... dear god...
If you thought Katie being dragged down the hall was scary... hoo boy. Kristi gets not only dragged out of Hunter's room in a similiar manner, latching onto the railing for dear life, but runs back into Hunter's room only to be dragged away AGAIN, this time down the stairs, through the hall and into the basement after which the door SLAMS shut after her. Later we're even graced with a look at the back of the door showing that she appears to have tried to claw her way out of the basement.
The Nightmare Fuel gets worse when you realize the scratches aren't from her attempting to escape, and were put there purposefully and probably not by Kristi herself, considering they say "MEUS." That's Latin for "mine." It seems Toby the demon is rather... possessive.
The scene where Ali's asleep on the couch, and the TV suddenly goes to static while a shadow moves over her, as if the demon's going to consume her, before she wakes up, the shadow disappears and the TV goes back to normal. Especially bad if you tend to fall asleep on your own couch with the TV on.
In the very last scene, Hunter is wailing and crying after possessed-Katie kills Kristi, but, as Katie carries him away, he starts laughing.
The Third Movie
So the trailer starts off reasonably, right? It cuts between young Katie and Kristi and stock footage from the first two movies. Then it cuts to them in the bathroom attempting to summon Bloody Mary. It doesn't seem to work, and they leave the bathroom, oblivious to a shadowed figure who looked suspiciously like Possessed!Katie standing right there in the mirror, watching them walk away. The audience only sees a shadowy figure, but it's still pretty creepy.
While the aforementioned scene never takes place in the movie, the movie does have, if you will, its own version. Randy stays home with Katie while Dennis and Julie take Kristi to the hospital for a fever. Katie wants to play Bloody Mary. Once they turn the lights back on, Randy has a scratch on his stomach. The demon then starts slamming on the other side of the door. By now, Randy and Katie are thoroughly terrified. Slowly, Randy opens the door and the demon throws Kristi's tea party furniture across the room. After this, Randy packs up his belongings, tells Dennis to watch the tape, and that, in a nice way, he is completely fucked.
Following the earthquake scene is the shot of the dust from the ceiling covering the demon. -shudder-
Any scene with the oscillating (side-to-side) fan:
The babysitter jumps in front of it for a "gotcha!" scare.
Camera pans left. White sheet on top of, presumably, one of the kids. Camera pans right, the babysitter is sitting quiety. Camera pans left, the sheet-wearing kid is gone. Camera pans right, the sheet-wearing kid is standing right behind the babysitter. As the camera begins to pan back to the left, the sheet drops and you realize it wasn't one of the kids.
Camera pans left, as the mother walks over to the front door in response to someone knocking on it. naturally, being a ghost movie, there's nobody there. She walks back to the right, and slows down in shock as the camera pans back to the right, and we see that everything that isn't bolted down is gone. This includes the table. There's a beat, and everything comes crashing down, having somehow been suspended above-camera.
Kristi decides that she doesn't want to play with her imaginary friend any more. He is not happy about this. The following night-time scene, we see objects in Katie and Kristi's room coming to life. The demon throws Katie's blanket at the camera, moving the bed around, and forcibly dragging her out of bed into a cubby hole. The demon stops tormenting Katie when Kristi agrees to...something.
The final night. That's all that needs to be said.
The old ladies. They were billed as "creepy ladies" for a damn good reason.
When Katie bumps into Toby. Toby doesn't like to be touched.
Maybe more of a fridge horror but the demon goes under 9 year old Katie's covers. And disappears inside of her. Say whatever you want about this but it's simply terrifying.
The first movie opens with a card explaining that the producers edited from found footage. One might wonder why they did, until one looks at the box office revenues. The second move is edited from Kristi's home security camera footage, and while no one takes credit for it, Ali survived and might have approved it. The third movie is edited together from the old VHS tapes Katie got from grandma. But who did it? The tapes disappeared from Kristi's basement during the events of Paranormal Activity 2, implying that the demon or Possessed!Katie has them. So who edited PA 3? And why are they showing it to us?
The Fourth Movie
The trailer alone already gives one of the most horryfying moments of the entire franchise. Alex is talking to her boyfriend and filming with her camera, and she then spots Creepy Child Hunter standing outside her house...and then he disappears...lights go off and then...Possessed!Katie's silhouette appears behind Alex, the door closes behind her and then...nothing.
Also, if you look close enough, you can see Hunter is emitting two shadows.
The knife violently crashing from above into the kitchen counter immediately after Doug moved away from it.
Alex being levitated into the air for no particular reason unknowingly as she's sleeping.
Wyatt waking up in the middle of the night into the living room where the Kinect's night vision is, talking to the demon, and a shadowy child like figure is right behind him. It is CREEPY.
The Final twenty minutes or so... holy crap. The mom is all alone in the house and hears strange noises then, in a callback the demon starts BREATHING on her and then she's dragged into the air and thrown around the room. Then Katie appears and hides her body Later, Ben arrives at the house and hears strange noises and investigates upstairs. He sits in Alex's room and begins using her computer... when Katie appears RIGHT BEHIND HIM. He starts to turn around and she brutally breaks his neck and stashes him in Alex's closet. When Alex returns, she calls his phone and the ringtone from his comes out of her closet. She begins to investigate when an invisible force throws her across the room and drags out into the hall. She goes to find her dad, who followed Katie, who took Wyatt, into her house. inside the house her father is attacked by the unseen force and dragged down a hall. As Alex follows him Katie steps out into the hall and Alex begins to beg for her life. Suddenly, Katie begins charging at Alex and her face contorts demonically and Alex escapes through a window. She spots Wyatt and begs him to leave with her but she turns around and dozens, maybe hundreds of blank faced people walk towards her. She turns around and possessed!Katie attacks her... and that's the end.
Robbie is one hell of a Creepy Child. Throughout the movie, whenever he shows up, the hauntings take a turn for the worse.
Not to mention his weird friendship with Wyatt and their "imaginary friend."
The garage scene.
"He looks just like his mother."
While Wyatt is in the bathtub and his mother downstairs talking on the phone, the demon thought this would be the perfect time to actually drown the poor boy.
The moving chair and tricycle.
Three words: Circle inside triangle.
Non-demon example: At one point, Alex almost gets hit by a car while crossing the street.
Any scene that happens to take place in the house across the street from Alex's house.
The Marked Ones
Oscar's body hitting the top of the car.
The cleansing scene, culminating in Jesse bursting from out of nowhere after a moment of silence while Hector sees through the night vision what appears to be the wall rippling and shrinking.
Jesse's demon face.
The increasingly antagonistic nature of the Simon game.
When Jesse goes to Anna's basement by himself. He investigates the basement searching for his dog, runs into the children versions of Kate and Kristi with possessed eyes, tries to run away only to find himself trapped, tries calling for help and behind one of the plastic sheets, a shadow can be seen approaching him...and suddenly charges at him with a ungodly roar.
As is par for the course of the series, the last scene. Hector, Arturo, Santo and Marisol go to Kate's grandma's house from the 3rd movie to rescue Jesse, and are attacked by the witches while trying to get in. Santo is killed and Arturo is fending them off with a shotgun while Marisol and Hector go inside trying to survive and find Jesse. Hector hears Arturo calling out to him from outside and something is slammed against the window from the outside, Hector turns around and finds Marisol is nowhere to be seen, tries to find her and get out of the house only to find himself surrounded by witches at every corner. As he continues to venture through the house, Marisol suddenly falls from a hole in the ceiling, dead. AND finally, Hector is facing the living room when a demon roars and a sheet is flung at him, revealing a possessed Jesse charging at him. He runs away and locks himself in the one room with barricaded windows and as Demon!Jesse is barging in, Hector's one chance of escaping is an odd door with runes around the frames. This somehow sends him back to the moment Kate kills Micah in the first film and when he tries to escape...he is ambushed by Demon!Jesse and the camera falls down, ending with Demon!Kate turning off the camera while staring directly at it.