The Time Meddler is just a really strong story. The side characters (well, one of them, Edith), are well done, the sets are really good, and the main cast is stellar. It's also a fascinating story in terms of establishing Doctor Who lore - we learn for the first time that the Doctor is, in fact, alien, and from the same place as the Monk, and we even learn that the Doctor's fancy TARDIS isn't all that.

Speaking of the Monk - absolute highlight of the story. He plays off the Doctor really well, offering an opposing but nevertheless similar character. What a guy