The "soul-sucking" portion of the Revenge of the Mummy roller coasters, at least in the Orlando design: you're sent up a hill where Imhotep's massive face is projected in front of you, screaming "Your souls are mine!" and it suddenly launches the car at 45 miles per hour up the incline and through his mouth with a massive roar.
There's also the first scene with Reggie's soul getting sucked out.
And the part with the scarab beetles. The ride gives off the illusion of thousands of them crawling all over the place...
Plus the scene when Imhotep's minions pop out of nowhere. (pictured right)
Did we mention the part where you're going high-speed in almost complete darkness and whizzing past freaky mummy spirits and people getting tortured? Most would agree this ride reigns supreme as far as scare factor goes. It's heavily implied that this is the Egyptian Underworld. — more specifically, it's Egyptian Hell.
Oh yeah, and at the Hollywood version, when you're exiting, one of the mummy statues you walk past isn't actually a statue... and will reveal that fact to a certain guest at random.
At the beginning of the ride, it is revealed that the shark has returned, as it sinks and eats the passengers of another boat. You get to hear the not-so pleasant sound of that boat's skipper begging for help and then screaming over the radio transmission.
There's also a scene where the boat enters a dark boathouse, which suddenly begins to crumble apart due to the shark attempting to break in. It succeeds, pops out of the water, and lunges right at you. All in the dark.
Additionally, upon entering the boathouse, you are first treated to the sight of some gory pieces of flesh from another shark lying on the floor.
After the boathouse, the shark lunges right out again to the left of the boat, creating a rather effective jump scare. In the original version of the ride, it actually bit into the boat and dragged it a couple of feet before the skipper eventually regained control.
The ride's gruesome ending where the shark accidentally bites a power line, electrocuting and then completely frying it. You even see its roasted remains, until it reveals that it's actually still alive when it makes one final lunge towards the boat before being killed by the blast of the skipper's grenade launcher.
The ride's original ending was arguably even more graphic. The skipper would shoot a grenade right into the shark's mouth. The shark would then go back underwater, only to explode mere seconds later, leaving behind a pool of blood and pieces of skin and flesh.
Jurassic Park River Adventure
Right from the start of the ride, even the herbivorous dinosaurs are a bit unsettling, especially the Stegosaurus that suddenly shows up to the right of the boat.
The Dilophosaurus is depicted here in all its glory, first acting all cute and innocent, and then switching to ferocious as it spits its venom at you (even if it's just water).
Velociraptors popping out of the dark at you when you're inside the building. The Hollywood version even has a raptor that pops out at you from the ceiling.
Also at Hollywood, they amplify this 10x during Halloween Horror Nights by placing statues of Velociraptors brutally ripping JP scientists apart and lighting it all up with strobes.
And of course, there's the massively imposing Tyrannosaurus itself getting so close to the vehicle that you could almost touch it, before you're sent down in 85 foot drop.
Earthquake: The Big One
Even the minutes before the ride even started was quite chilling, the ambient subway noise in particular succeeded in creating a feeling of absolute dread in regards to what you knew was about to happen.
The unsettling voice of the subway announcer combined with the lights turning off one-by-one as the vehicle began to move forward also added much to the creepy factor.
The absolute chaos of the earthquake scene itself, the unpredictably of it from the perspective of a first time rider made it even more scary.
There are several moments that stood out in particular, such as the ground above you coming apart, which results in a giant gas truck coming down and nearly exploding in your face. There was also a moment when an incoming subway train on the other side de-railed and almost collided with the one you're in. Bear in mind that this all happened while you could hear the sound of people screaming in terror.
There's a scene where Shrek teases Donkey with a spider dangling from a web. Freaked out, Donkey blows the spider towards the audience (do remember this is in 3-D!), only for dozens of spiders to suddenly and randomly drop down from the trees and land on the ground. The seats in the theater then utilize a "leg tickler" effect to make it seem like the spiders are crawling all over the audience's legs. Arachnophobes beware.
The stone dragon Farquaad's ghost uses in an attempt to kill the two titular heroes. Remember "Dragon" from the first movie and how vicious she was before falling in love with Donkey? Picture that but ten times worse. It's also worth mentioning that its roar quite literally is so loud that it shakes the entire theater and you are given some rather unpleasant close-ups of it (in 3-D!).
One particularly memorable thing about the attraction is that the whole pre-show room is themed as a torture chamber, complete with dozens of classic torture devices positioned all around you, including right above you.
There's also another disturbing little detail to the pre-show room. While you are walking into the main show room, listen very closely to the background noise. You can faintly hear people being tortured and screaming for help.
Kongfrontation (gone but not forgotten) had you going face to face with the giant ape himself. And we mean that literally. He was a giant animatronic that was really right in your face. In addition to all that, the ride would simulate the feeling of him picking up the vehicle you're in...and dropping it.
The dementors and giant spiders in Harry Potter and the Forbidden Journey, especially with how close to them the ride puts you.
E.T. Adventure is a great example of unintentional nightmare fuel. Firstly, the animatronic policemen are heavy in the Uncanny Valley department. Plus E.T.'s friends on the Green Planet look rather...disturbing, to say the least.
The Tyrannosaurus in the Back to the Future ride, specially with how you get swallowed by it.
"Poultra" in the former Jimmy Neutron's Nicktoon Blast ride. A giant chicken monster-thing with corrosive green breath and loud footsteps that violently shake the vehicle.
The scenes in The Amazing Adventures of Spider-Man where Electro electrocutes the vehicle you're in and when Scream jumps on the vehicle and basically attempts to claw your whole face off.
All of the Decepticon encounters in the Transformers ride. The Decepticons are already scary as is, but when you add in 3D effects and intense ride movements, ho boy...
The many technical malfunctions that have plagued Hollywood Rip Ride Rockit since its (delayed) opening. One such malfunction left a batch of riders stranded on the first lift hill for over two hours (and mind you, the first lift goes up at a near 90 degree angle).