"The Girl in the Fireplace" -- Series 2, 2006
Not only is "The Girl in the Fireplace" possibly the best episode of Doctor Who to date, it's quite possibly the best episode of television I've ever seen. It's a very beautiful standalone episode that manages to pull off funny and hearbreaking, romantic and scary. The Doctor falls in love, but time itself snatches his love away from him.
The Doctor, along with his companions Mickey and Rose, lands on a derelict ship floating in space. While exploring, the Doctor finds an eighteenth century fireplace inexplicably placed among the futuristic technology of the ship. Upon travelling through, he discovers that he is transported to pre-revolutionary France, and he befriends a child Madame de Pompadour, Reinette, after rescuing her from a mysterious figure made entirely out of clockwork. The clockwork creatures need her brain in order to restore the ship back to its former glory, having harvested the rest of the crew for parts. The androids have opened time windows into her life throughout the ship, and are trying to find her at the age of thirty-seven, when they believe her brain would be compatible with the thirty-seven year old ship.
The Doctor travels back and forth between the fireplace and the ship, only discovering that each time he returns back through the fireplace, time in France has passed dramatically. A moment in the real ship could be years in France, causing the Doctor to finally lose his growing romance with Reinette as the time window closes forever.
It's a heartbreaking episode which demonstrates that the Doctor will eventually end up alone, developing the sadness of the character, as perfectly demonstrated by Tennant. It also has a neat twist at the end (which I won't reveal) that makes the episode all the more surreal.
It, like "Blink" and "Silence in the Library"/"Forest of the Dead" is a Moffat-penned episode, making series 5 of the show look all the more brighter with Moffat as showrunner. And even though Tennant, who portrayed the Doctor throughout the five episodes listed, will not be portraying the Doctor, he leaves behind a great legacy as the Tenth Doctor.
Good list, but I still think Human Nature/ The Family of Blood beats all of these.
"Planet of the Ood" was another of my favorite Tennant episodes for the way it dealt with the theme of slavery, and for the way it further fleshed out Donna Noble's character beyond her brash and boisterous persona, showing a more tender and caring side. Besides that, who doesn't love the Ood?
I agree that “Human Nature/ Family of Blood should be on this list, over Waters of Mars. And I would also replace midnight with The Empty Child