10. The X-Files - How The Ghosts Stole Christmas
This is a Christmas caper with a classic X-Files twist: it's set entirely inside a haunted house. A brilliant fusion of the festive and the spooky, the episode sees Mulder and Scully investigating a mansion rumoured to be haunted by two young lovers who committed suicide together so they'd never be alone.
These two spectral sweethearts are played by comedy veterans Ed Asner and Lily Tomlin, and they're the only cast members aside from Duchovny and Anderson, meaning this episode has the smallest cast of any X-File with just four characters total.
The minimal casting extends to the location, with the majority of the episode set entirely within the haunted house's library. Smartly however, the library is split into multiple versions of itself, allowing Mulder and Scully to be separated from each other. Attempting to leave simply brings them to another instance of the library, so the agents are forced to come face to face with the snarky spectres.
It's a brilliantly economical premise, and it makes for a heartfelt horror about love and loneliness. The themes are encapsulated in the bittersweet fan-favourite final scene when, having finally escaped from the haunted house, Mulder and Scully exchange Christmas presents. This reinforces their care for one another, but also acts as a grim mirror of the ghosts, whose devotion to each other isolated them from everyone else.