10 Best One-Time Only Doctor Who Villains

10. The Gelth (The Unquiet Dead)

The Gelth are among the most underrated baddies in Who's modern era.

Appearing for the first and only time in the Ninth Doctor episode The Unquiet Dead, the most intriguing thing about them is that they're not technically villains, a layer that makes them an interesting creature to use as the story's de facto antagonists.

Appearing like blue, gaseous ghosts, the Gelth are actually a humanoid species who have suffered a great deal. After losing their bodies during the Time War, they're essentially seeking refuge in this episode, and their only desire is to inhabit human bodies and take a physical form once again.

So when they start possessing corpses (and start attacking Rose with said corpses), and when billions of Gelth try to squeeze through the rift and take the Earth by force, they're acting from a place of desperation. We won't go as far as saying they're relatable (we can't speak for anyone else, but we've never lost our bodies during a Time War) but we understand why they felt like they needed to act in a villainous way.

It's rare for a Doctor Who villain to have such a compelling motivation. Plus, their blue ghostly forms gave them a really cool, mesmerising look too.