The greatest alien visitors in science fiction are totally invulnerable — except for one crucial weakness. And the best almost-unstoppable aliens have a weakness that is more poetic than Sylvia Plath and William Blake put together. Just look at our video compilation of aliens encountering their most poetic Achilles heels, and then check out our complete round-up.

Aliens Should Always Have Poetic WeaknessesS

Superman.
He's the last survivor (or one of the half-dozen last survivors) of the exploding planet Krypton. And he's invulnerable to just about everything in the universe, including nuclear bombs and the vacuum of interstellar space — but he can't come anywhere near a radioactive fragment of his own planet without dying. Or, if it's a red fragment, it'll turn him into a dwarf or a dragon. Of course. Also, Superman's pal, the Martian Manhunter, has a terrible vulnerability to fire — but it turns out to be mostly psychological.
Why it's poetic: Come on, he's lost his home planet... and now whenever he encounters part of it, it nearly kills him. The loneliness, the desolation.

Sontarans.
On Doctor Who, the Sontarans are cloned super-soldiers from the distant planet Sontar. They're almost unstoppable (although in their latest appearance they turned out to be pretty darn stoppable once you used non-copper bullets.) And their only weak spot is a small vent in the backs of their necks, which they use to recharge.
Why it's poetic: They're super-warriors, so they must always face their enemies. I mean, they could put a cap or a shield onto their neck-holes, but they choose not to. Because they need their fatal flaw to remind them who they are.

Aliens Should Always Have Poetic WeaknessesS

The Klowns.
In Killer Klowns From Outer Space. It turns out you can kill a killer klown by popping their red nose — it makes perfect sense!
Why it's poetic: They wear their most vulnerable part right in front of them, so they can see any attacks coming. Plus, it's like slapstick and murder rolled into one. Dude!

The Martians.
In War Of The Worlds, the invaders can clobber everything that humans can throw at them, and they scoff at our huge weapons systems. But then they're felled by the smallest enemy of all, the common cold.
Why it's poetic: Mostly because H.G. Wells gets so fancy and flowery talking about the "smallest and humblest of all God's creatures" and how it stomped the monsters' asses. (How does he know germs are humble?)

Aliens Should Always Have Poetic WeaknessesS

The Fithp
The Fithp are sort of weird super-intelligent elephants who use superior, if borrowed, technology to invade Earth in the 1986 novel Footfall, by Larry Niven and Jerry Pournelle. The humans are hopelessly outclassed, but they have one advantage. In the Fithp culture, when two herds fight, one eventually surrenders and gets absorbed into the other herd — so they're not prepared for humans to surrender and then mount a resistance or plan sabotage.
Why it's poetic: Because these super-elephant guys fail to understand the most human of behaviors... sneakiness.

The Colonists
In The X-Files, the aliens seeking to invade the Earth create super-soldiers who have only one weakness: their bodies are torn apart by the magnetic fields of large deposites of magnetite.
Why it's poetic: The alien soldiers are super-human because of their metallic bodies — but those same bodies make them vulnerable to magnetite. Woah.

The Crawling Eye.
Aliens who are basically just huge eyeballs with tentacles invade the Earth and nothing can stop them — until one human figures out the aliens have no defense against the awesome power of fire!
Why it's poetic: Because the eyes are burning! It's a tremendous metaphor for the blindness of power. Or maybe it's just a metaphor for how much you'll be rubbing your eyes with sleepiness as you try to pay attention to this movie.

The Signs invaders.
We've already talked about this a fair bit, but the aliens who decide to attack/invade/kidnap kids in M. Night Shyamalan's Signs have a terrible vulnerability to plain old water.
Why it's poetic: The humblest of beverages! Or maybe, the fact that the aliens can't protect themselves against water without giving up their shape-shifting abilities. So they rely on the chameleon thing, to the exclusion of protecting themselves.

The Alien Teachers
Aliens replace the teachers at Henderson High School in Robert Rodriguez's The Faculty. And it turns out the aliens' only weakness is Zeke the drug dealer's "homebake."
Why it's poetic: It's the humblest of drugs! Oh, wait. I mean, come on. They're impersonating teachers, and they're vulnerable to the students' drugs. That's awesome. Plus, it's proof that drugs really are good for you. And the school drug dealer is your friend. Etc.

Leto Atreides II
In Frank Herbert's God Emperor Of Dune, Leto lives for 3,000 years and becomes nearly unkillable because he's part sandworm. But then it turns out that he's gained the sandworms' vulnerability to water.
Why it's poetic: He inherits the weaknesses as well as the strengths of the worms. Dude, come on!

Aliens Should Always Have Poetic Weaknesses

Stitch:
Stitch, from Lilo and Stitch, is a super-awesome alien koala creature. Except that he can't swim.
Why it's poetic: Stitch's super-dense body makes him indestructible, but also means he sinks like a rock. Oh noes!

The Tenctonese.
The aliens from Alien Nation could be burned, and even killed, if they came into contact with salt water. What is it with aliens and water of various types? (Thanks Roraz!) Science fiction author Robert J. Sawyer has an incredibly complicated explanation of how the Tenctonese's weakness actually makes sense.
Why it's poetic: You can't cry on their shoulders... or if you do, they'll definitely feel your pain.
Note: In the course of putting this blog post together, I found this post at Everything2, which was pretty helpful in coming up with some examples.