8. The Parasite Slither
What Its Deal Is: The Parasite that infects Grant Grant (Michael Rooker) in Slither is a small and unassuming thing that lands on Earth in a scene referencing 1958s The Blob. It has the ability to fully control its host; it can also create further parasites that infect the population allowing the Parasite to control them all at once. Instead of Zombies, think of them as one giant super-zombie broken down into itty bitty pieces.
Why Itd Be Unstoppable: As detailed in a few clever flashback sequences, The Parasite retains certain qualities of the host; from size and shape to cognitive ability, and with human hosts it even retains some of the personality. Thats why its weakness in Slither is the same as Grants weakess Starla (Elizabeth Banks). In reality, this weakness would be its greatest strength. The flashbacks explain that the previous hosts for the world consuming Parasite had been little more than galactic animals. But for the first time ever, this Parasite would have had access to all of the intelligence, the ingenuity and the adaptability that the human mind is capable of for that reason, we really would have been f*cked. Mind-Controlling World Destroying Parasite + Human Sentience and Ingenuity = End of Days, no matter which way you look at it.