14. City Of Death (Fourth Doctor)
This is certainly one of the most unique Dr. Who stories ever attempted, and also held the distinction of being the most watched Who story until the 2008 Christmas Special. Let's see how quickly I can summarize the plot: An alien named Scaroth got split into living in six different timelines simultaneously, and in one of those timelines steals the Mona Lisa to fund experiments in time travel. This story in general feels more like a segment of The Hitchhiker's Guide To the Galaxy, with a mind-bending story, bizarre humor, and some awesome surprise cameos in the last of its four episodes.
13. The Girl In The Fireplace (Tenth Doctor)
Season 2 of the revived series was certainly unique, in that nearly every episode was either critically hailed (School Reunion, Rise of the Cybermen, Army of Ghosts, The Impossible Planet) or universally hated (New Earth, Fear Her,
The Idiot's Lantern). But no episode received as much praise as The Girl In the Fireplace. This story introduced the idea of two people trying to communicate while being caught in different time streams. While this idea has been beaten to death lately (for instance, in The Eleventh Hour or the River Song story arc), in this episode it felt fresh and experimental, and generally brilliant. When Rose, Mickey, and the Doctor land on an abandoned spaceship adrift, all looks fairly normal for an abandoned spaceship adrift. Until they notice massive amounts of energy being expended keeping a temporal doorway open to 18th century France. It turns out there are robots on the ship that are watching the life of Madame de Pompadour. Trust me, it makes more sense in the episode. Just watch it already.