What is the scariest science fiction or fantasy film of all time? Which classic horror movies stand above the rest, as the most terrifying?

The staff at io9 has painstakingly selected the best of the best, and compiled our ultimate list of the scariest movies of all time. And now, check out numbers 11 through 20...

20. The Ruins (2008)
What: A pack of kids on Spring Break go off the map and into the wilderness, looking for an ancient temple covered with a strange and unusual plant. But once they've touched the ruins a band of locals surround the teens and won't let them off the sacred stones. Meanwhile the greenery is slowly picking each one off one by one. Trapped, the gang starts to unravel.
Why: First of all, this movie made plants scary — no easy task. But the real meat of this movie isn't tied to the carnivorous plants. Rather, the horror is slowly revealed as each youth loses his or her mind. You see, the plants start burrowing inside a few of their open wounds, writhing about under their skin and slowly ingesting them from the inside, and once they figure this out it's self mutilation time.
Scariest Scene: When the little blonde takes the knife to her body a little too aggressively.

19. Cabin Fever (2002)
What: Eli Roth's big fat debut picture which rocketed him into horror movie stardom. Looking for a little woodland coitus, a pack of kids head to a little cabin in the woods to work out their genitals. They are quickly overcome by a strange flesh-eating virus that starts peeling away their skin.
Why: A perfect blend of comedy and gore! Plus this movie was a welcome return to David Cronenberg-style terrors, of what happens when your body falls to pieces.
Scariest Scene: Eli Roth's dog Dr. Mambo eats the lips off Jordan Ladd's face.

18. Insidious (2011)
What: What starts out as a Haunted House flick twists quickly into a haunted kid flick. A little boy falls into a "coma" and his body sparks a bidding war among all the disembodied spirits of the world.
Why: Full disclosure, we almost kicked this movie off the list for having one of the most anti-climatic endings of all time. That said, the majority of this film turned us into pools of quivering jelly. Holy hell, director James Wan is absolutely relentless with his scares. Jump scares, slow burns, creepy creatures sitting in the corner of the room... nothing is safe. Even the titles are haunted by paranormal goings-on.
Scariest Scene: Even though the jump scares were scary, we were even more terrified by the lurking little boy in the corner of the laundry room.

17. Friday The 13th (1980)
What:Okay, okay, okay — we know what you're going to say, it was the MOM, and that's not science fiction or fantasy. We get it. HOWEVER, the final scene of Friday The 13th, where baby Jason claws out of the water and attacks the final survivor (whether it's a dream or not) forever set up this film as a fantasy franchise, because it proved that you could bring back Jason in some form or another. And he was all but impossible to kill or even maim.
Why: A relentless killer that nothing can stop. It doesn't get much better than that.
Scariest Scene: The canoe surprise!

16. 28 Days Later (2002)
What: A group of animal-rights activists unleash the Rage virus on the world. With a five-second incubation time, the world easily crumbles. But the audience never actually gets to see the fall of civilization, instead we wake up with a young Cillian Murphy (who's been stuck in a coma for 28 days) and view the entire destruction from his POV.
Why: Danny Boyle upgraded the shambling zombie we were all familiar with to a adrenaline crazed rage machine.
Scariest Scene: While the chase scenes are pretty intense, as was the big showdown with the crazed military folk, this movie will always be remembered for the epic opening when Jim walks through a completely abandoned London screaming out for help.

15. Blair Witch Project (1999)
What: The pioneer of the found footage genre, Blair Witch follows a three-person camera crew into the forrest of Burkittsville, Maryland to make a documentary about a local legend, the Blair Witch. But the witch finds them first.
Why: Forget the fact that this movie was made on a shoe-string budget and introduced most of the world to the shaky-cam. This movie is scary. It does so much with so little: the sounds of a baby outside of the tent, piles of rocks, a bloody bit of shirt from a missing crew member. The mind is driven wild with possible death scenarios for each of the Witch's victims.
Scariest Scene: When one crew member finds Josh's shirt and the body parts the Blair Witch lovingly tied up and dropped at the front door of their tent.

14. The Haunting (1963)
What: A group of strangers (all of them having dalliances with the supernatural) are gathered together at Hill House, and observed by a team of paranormal investigators. Things go terribly wrong.
Why: This movie is horrifying and just about every single creep-out still stands up today (even though the 1990s remake tried to kill it). The Haunting uses sound to drive the audience mad. Mysterious beings would bang on the doors and strange voices could be heard inside the walls. Shadowy faces creep across the screen to the point that you start seeing things that were never on the screen in the first place.
Scariest Scene: When Eleanor realizes she been holding a spirit's hand!

13. Nightmare on Elm Street (1984)
What: A man with fingers made out of knives and a charred face chases down the teens on Elm Street, murdering them in their dreams. But when these kids die in their sleep, they also die in real life.
Why: This film established one of horror's sassiest monsters, Freddy Krueger, with buckets and buckets of blood. No place is safe from his claws — not even the bathtub. Krueger and director Wes Craven got crazy creative with their teen slaughters, and the crazy pays off.
Scariest Scene: Watching Tina get sliced and diced while dancing on the ceiling.

12. Event Horizon (1997)
What: In the future Dr. William Weir (Sam Neill) invents a space ship that can bend reality as we know it by opening up a portal to another world to shave off years on interstellar travel. Sadly, that portal he opened leads to Hell.
Why: Hell in space! The Event Horizon uses the crew's worst fears to knock them off one at a time — including the eyeless dead wife of Dr. Weir.
Scariest Scene: When Weir goes full crazy and rips out his own eyes, because where they're going they won't need them...

11. Carrie (1976)
What: The sad, loner girl in high school develops psionic powers, and after getting pushed to the edge, uses them to murder her class. Directed by Brian De Palma and based on the Stephen King novel, this movie is a staple.
Why: You root for Carrie, played by the amazing Sissy Spacek. Even when she's stabbing her terrible religious nut-job mother to bits, all these assholes kind of had it coming to them in the end. Then again there are a few gems in this cesspool, but their good intentions are marred by a bucket of pigs blood.
Scariest Scene: The breezy beautiful dance to the podium Carrie makes as she's crowned prom queen, followed quickly by the brutal execution of her classmates.

io9's 50 Scariest Movies Of All Time: 50 - 41
io9's 50 Scariest Movies Of All Time: 40-31
io9's 50 Scariest Movies Of All Time: 30-21