The Scariest Aliens Ever from Sci-Fi Films

They Came from Outer Space

When it comes to scary aliens, science fiction has never failed us. From the xenomorphs of the "Alien" films to the titular Killer Klowns from Outer Space, the sci-fi universe is chock full of terrifying extraterrestrials to choose from.

But not all alien monsters max out our "scare" meter at Space.com. Check out our picks for the scariest movie aliens of all time!

(Editor's note: Just in case you haven't seen any of these movies, we'll put their names in the preceding slide so, you know, spoiler alert.)

First Stop: The Kaiju of "Pacific Rim"

'Pacific Rim's' Kaiju/Precursors

In 2013's "Pacific Rim" giant Kaiju monsters invade Earth as part of a colonization plan by alien Precursors from another universe. Any alien race that can create a seemingly unending army of diverse (and ginormous) monsters to invade another planet is one to be feared. And we love giant monsters (see Cloverfield later), which is why Pacific Rim's Kaiju get a nod here. But there's a case to be made that creatures from another dimension might technically not be aliens, so we'll bring them at last for now.

NEXT: "Battle: Los Angeles"

'Battle: Los Angeles' Invaders

In the "Independence Day" films, alien spaceships make grand entrances before destroying cities, but the invaders in "Battle: Los Angeles" make no such entrance. They literally fall out of the sky and hit the ground running with ungainly bodies that look like a hybrid of tech and organic matter. But it’s the clear military discipline of these fighters that earn them a spot on our list.

NEXT: Space gorillas from 'Attack the Block'

'Attack the Block's' Space Gorillas

Yes, 2011's "Attack the Block" is a science fiction comedy film, but the unrelenting nature of its gorilla-like aliens warrants a mention here. They're big, covered in black, spiky fur and have fang-filled mouths that glow with an eerie blue hue. They also apparently use Earth as breeding ground, so don't get between a spiky space gorilla and its mate.

NEXT: 'The World's End' Invaders

'The World's End' Invaders

Another comedy? You bet. In 2013's "The World's End" Simon Pegg, Nick Frost and friends stumble into an invasion during a pub crawl in their hometown. This film brings us our first human replacement entry, with androids who replace unsuspecting citizens and report to an alien entity known as The Network. Getting replaced by an android doesn't sound pleasant, and points to "The World's End" for its creepy glowing eyes and mouths when the androids attack.

NEXT: "Monsters"

Monsters

Monsters is a travel story full of nightmares. A photojournalist tries to bring his employer's daughter, Samantha, safely through the United States via Mexico -- where, unluckily for the duo, a bunch of tentacled monsters live. These monsters are so brave that they fiercely attack armed guards, and even breach the United States border. The movie goes beyond portraying these creatures as "monsters", however, as it also shows how they communicate via light pulses.

NEXT: "The Puppetmasters"

The Puppetmasters

This classic from Robert A. Heinlein goes all Cold War on readers, showing what happens when American secret agents fight slugs from space. What makes these creatures truly terrifying is their ability to take over a human, making use of their skills to execute complex tasks and to allow more slugs into the local environment. In a pretty prescient foreshadowing from 1951, Heinlein also discusses what planets these creatures could have arrived from, which is similar to today's discussions of potentially habitable environments on exoplanets.

NEXT: Edgar the Bug from "Men in Black"

Edgar the Bug from Men in Black

This is every horrible cockroach you've ever seen, but a thousand times grosser and many times bigger. Worse, he can even disguise himself as a human being. He's smart, crawls on walls and even camouflage his voice. Getting rid of him will take way more than a little bug spray. Extra props to screenwriters for making Edgar a voice for marginialized insects everywhere when he says, "You ever pulled the wings off a fly? You care to see a fly get even?"

NEXT: "Killer Klowns from Outer Space"

Killer Klowns for Outer Space

This isn't a movie to watch if you're afraid of clowns, because these clowns are even worse -- they're evil aliens! Their cocoons look like cotton candy, they use old-school rayguns to hunt down innocent townspeople, and worst, they enjoy eating the liquefied remains of the people killed in their rampages. Oddly, this film is a comedy, despite the awfulness of its monsters.

NEXT: The Sarlacc in "Return of the Jedi"

The Sarlacc Return of the Jedi

Many "Star Wars" fans may not know its name, but the sarlacc is memorable enough by appearance. It's that big looming black hole-type of a monster that's underneath the platform where prisoners Luke Skywalker and Han Solo are being sentenced to death in the 1983 movie "Return of the Jedi". The 1997 remastered version of the film even included tentacles, so -- best not to get too close to this hungry mouth.

NEXT: "The Fourth Kind" aliens

The Fourth Kind Aliens

Although this film is a mockumentary talking about humans being abducted by aliens, the aliens don't appear to be pleasant captors at all. The aliens speak ancient Sumerian (not being bothered to catch up with human language evolution) and perform barbaric rituals on the humans they capture. But whether these accounts are really true, the movie adds, is up to you to decide.

NEXT: "Event Horizon" Force