3. "Gridlock"
When I bring up the episode "Gridlock" to other Whovians, I'm frequently met by a widening of the eyes an emphatic exclamation of "Oh yeah! I forgot about that one! It's great!" This episode might be the best most forgotten about episode of Doctor Who. "Gridlock" may slip so many viewers' minds because there's a lot of stuff going on: mood patches, an underground highway, gigantic alien crabs, the death of the Face of Boe, a cat dressed as a nun, and a seemingly endless cast of eccentric New Earth inhabitants. What holds all of it together, though, is well-paced storytelling, likeable characters, and a touching resolution. This episode also showcases, for the first time, Martha's gumption, which in turn exposes a sentimental nostalgia previously unseen from the Doctor. He gives us a description of Gallifrey before the hell of the Time War, and we see him soften a bit, remembering a long-lost love for his home planet. It's a beautiful moment to conclude a solid, well-rounded episode.