6. The Slitheen
They may not have been seen since 2005, but they have still featured in the Whoniverse since. Whether it be in the Sarah Jane Adventures or Doctor Who, all Slitheen stories have a clear basic plot: Destroy those on Earth and then sell the planet for a profit. They kill an authority figure at the beginning of an episode so the Slitheen can take over their appearance. They plan to destroy Earth, and then sell on. When they fail, they come for revenge in the next episode they're in. Therefore, there is no need for us to ever see them again. And if we do, even without spoilers, we'll know the basic storyline for the episode.