The Evil of the Daleks
The final episode of Troughton's first series acted as a kind of bookend to the opening story which also featured the Daleks. And while there is one episode in this serial still out there, we'd love to see the whole thing come back. It introduces both the Emperor of the Daleks and Victoria Watefield, who was one of the Doctor's few companions to herald from a time previous to the 20th Century.
But on top of that, the story itself is quite interesting in that it features the Doctor being forced to make the Daleks more like individuals by adding "The Human Factor". The result is, we think, much better than when the 10th Doctor attempted something similar in Evolution of the Daleks.