8. The Slitheen
Green space aliens with big eyes and saucer-shaped spaceships. The only thing that keeps the Slitheen distinct from cliche Martians is that they have fat bodies, not skinny ones. They masquerade as humans and laugh too much about taking over the world. Nothing about the Slitheen is actually scary, seeing as all the main characters seem to easily outrun or outwit them. The real scare in "Aliens of London" and "World War Three" was that the humans were dumb enough to authorize the use of nuclear weapons, which could have destroyed the planet if the Slitheen had succeeded. Rather than being scary, the Slitheen were comic relief, and an excuse to shoot on a low budget: in London, with only a few aliens in most scenes. They are memorable, but not for the reasons villains should be.