10 Best One-Time Only Doctor Who Villains
9. Satan (The Impossible Planet/The Satan Pit)
Series 2's The Impossible Planet and The Satan Pit is a classic "base under siege" story, with the Doctor and Rose - along with a team of human researchers and some Ood servants - becoming trapped inside a space station orbiting a black hole.
The main threat of these episodes is Satan - or rather, the beast that inspires the myth of Satan. We don't actually see this being in the flesh until the end of The Satan Pit, but it has a presence throughout the entire story as a sinister force manipulating the researchers and the Ood, turning them violent, causing chaos within the base.
And it's this aspect of the villain that makes it so compelling. Sure, the enormous, hulking devil creature is a monstrous sight to behold, but its ability to get inside people's heads, messing with them, turning close friends against each other... that's what makes this entity a great villain.
Psychological torment like this is very difficult for the Doctor to fight back against (and sure enough, he has a rather rough time in this story), and when a villain manages to put the Doctor on the back foot... they've done a pretty good job in our book.