2) The Silence (“The Eleventh Hour”-“The Wedding of River Song”, 2010-2011)
The Silence are a rogue faction of a religious order of nudists. And these naked maniacs (wearing holographic clothes) decide to kill the Doctor before he can come to the planet Trenzalore, lest he answer a message from the Time Lords, who are trapped in a pocket universe. (Because if the Doctor answers the message, the Time Lords will know it’s safe to return.)
Luckily, the Silence’s members include a slew of confessors who are the perfect assassins, because nobody can remember seeing them and they have superstrong mind-control powers.
So… sorted, yeah? Send the confessors with the forgetting powers after the Doctor, and he literally won’t see them coming. They shoot him in the head, case closed. Right?