Pinning these particular individuals down was a tricky task.
Don't get this writer wrong - there are plenty of horror villains who apparently undergo a mysterious boost in IQ in the gap between sequels. One can only wonder what sort of brain training complements an existence as a fully fledged horror antagonist.
With that being said, the traditional status quo found within the horror genre means that, as opposed to relying on improving their levels of intelligence to ultimately win out, many villains simply triumph due to a combination of their supernatural abilities, a physical refusal to just die or the simple yet enduringly iconic premise oft found within this genre: namely being "f*ck your feelings, this is a horror movie".
It's intensely unfair but if you don't like it, try Marvel.
Despite this gloomy state of affairs, some villains add a further depraved string to what is already an inconceivably twisted bow, demonstrating noticeably improved cunning with regard to their wicked schemes as they make their return in their respective franchises. While their intellect may have seemed formidable the first time around, viewers were in for a nasty shock when this crowd gleefully made their triumphant comeback.
10. Juno Kaplan - The Descent 2
This may constitute a mildly erroneous entry as the true villains of the wildly successful The Descent franchise are the crawlers. Well, that's if you can call animalistic, mindless pre-humanoids "villains" in the traditional sense of the word. They won't be monologuing gleefully to you about the brilliance of their evil plans, put it that way.
Juno Kaplan, on the other hand - Sarah's duplicitous friend from the original - was plenty villainous. Not only did she accidentally murder their dear friend Beth, leaving her for dead in the nightmarish darkness, she was also revealed to have had an affair with Sarah's late husband prior to his death. Audiences were practically cheering when Sarah crippled her with a pick-axe in the film's climactic sequence, seemingly leaving her to die screaming in the darkness.
While the quality fell off spectacularly in the jump to The Descent 2, what clearly did not were Juno's levels of intelligence. Already an accomplished caver and climber, she is revealed to have survived her injuries as a bona-fide Rambo-esque crawler killer underground; you simply don't do that unless you adapt to your surroundings in double time. Juno may be a terrible human being, but she was already as sharp as a whip - a factor that was only honed by her time underground with the abhorrent creatures.