6. "Turn Left"
"Turn Left" is generally recognized as a quality episode. But for me, it's the most genuinely creepy episode of Doctor Who. The show so often deals with the past and the future, but it rarely delves into the territory of "what might have been." The narrative that results is interesting, frightening, and staggering in its possibilities. Donna Noble is my favourite companion, and while she excels as the humorous and unrefined best friend, Catherine Tate brings Donna to a whole new level in "Turn Left." The contrast between the Donna we know made resilient and confident through her friendship with the Doctor, we see a woman of great potential who is downtrodden and struggling. This episode reveals like no other, the symbiotic relationship between the Doctor and his companions.