6. The Boneless
Aliens don't necessarily have to come from outer space. They can come from a different dimension entirely, and that's the case with the Boneless. While Doctor Who Who has ventured into alternate realities before, being invaded by creatures from a two dimensional world is a new one for the show, so the Boneless definitely get points for novelty. The idea that they can watch us and interact with us from the safety of their own, unexplored dimension makes them additionally chilling. Their cold, studying detachment and ability to kill almost at will helps that sense of creepiness, though their flickering, shifting appearance - stealing the images of the dead - also makes them quite uncanny. They're defeated fairly easily in the end, but their threat level before that really ramps up the tension, and they're one of very few creatures that can affect the inside of the TARDIS. Their effect on chase scenes just adds to their effectiveness. Chase scenes are a big part of Doctor Who - after all, it makes sense to run from evil aliens who are trying to kill you, doesn't it? However, when that creature can make it so that a door can't be opened, or can be opened when they need to go through it, the chase is rather stacked against you - making the Boneless even more threatening, while linking them to a Who tradition.