The Kantrofarri, better known as "dream crabs," are literally the stuff from your worst nightmares. These predatory creatures leap from the ceiling, wrap themselves around your face, and use telepathy to force you into a euphoric dream while they liquefy your brain for a snack. Yeesh!
They made their creepy debut in the 2014 Christmas special "Last Christmas" when The Doctor, Clara, and a gang of North Pole scientists are ambushed by what look like demon claws (and later saved by Santa Claus himself).
Later on, Clara herself was facehugged and trapped in a trance. Though the Twelfth Doctor attempted to communicate to her sublimally, the allure of a fantasy world where Clara's dead boyfriend was alive proved to be too much for her. Determined to save her, the Doctor sacrifices himself to a dream crab to launch a rescue mission into her subconscious. But, just when they think they are able to break the spell, the group realizes that it's pretty tough to distinguish reality from fiction.
The Kantrofarri have a lot going for them in the monstrosity department. Their grotesque appearance, ability to prey on a person's subconscious and their wicked defense mechanism of dragging their victims into an Inception rabbit hole containing multiple layers of dreams -- all while they are eating their brain -- earns them a spot on this list.
Sometimes, The Doctor has to make tough decisions, and all of the potential outcomes suck. Although he thinks he knows best, he makes an occasional mistake and inevitably makes the shittier decision. One example in the modern series of decisions gone horribly wrong came when Nine and Rose Tyler faced the Gelth. After the TARDIS "accidentally" landed them in 1869 Cardiff, the pair linked up with Charles Dickens, a funeral parlor owner named Gabriel Sneed, and his clairvoyant servant Gwyneth to solve a mystery. Together, they investigated a series of possessions that were turning corpses into the walking dead.
The Doctor used Gwyneth's powers to communicate with the unknown entities and discovered they were members of the Gelth, an alien race destroyed in the Time War. Their pleas for help allowed The Doctor to let his guilty conscience and (let's face it) ego get in the way, and so he agreed to open the rift in the basement of the funeral home. The Gelth would use the corpses as temporary vessels until they found a new planet, and innocent humans would stop getting murdered. Everyone ends up happy, right?
Of course, it was a dirty trick by the Gelth, who were much larger in number and ready to kill everyone on Earth so they could take over. Nine's decision led to the death of both Sneed and Gwyneth, who had volunteered to be a bridge for the race to cross over. Thankfully, that right old chap Charles Dickens helped draw the creatures out of the host bodies using gas, and Gwyneth blew them all to smithereens. But as far as terrifying villains go, alien-made zombies remain pretty unsettling.
Second in longevity only to another species of exterminators, the Cybermen have been a pain in ass since the end of the First Doctor's era. The metal cyborgs from Mondas (a twin planet of Earth) have one thing on their mind -- assimilation. They are the shell of former human beings who have been stripped of their emotions and are on a mission to make every human just like them, just because they believe they are superior.
The Doctor has tried to get rid of them, but like these guys are like Twitter trolls: They keep coming back. Cybermen are a ruthless clan who have done everything from being a catalyst for The Doctor's first regeneration to raising Clara Oswald's boyfriend Danny Pink from the dead, only to convert him and cause him to die again to save Earth. In other words, they are jerks.