7. The Evil Of The Daleks
This seven-part thriller showcased some chilling experiments the Daleks were conducting on humans, truly living up to the "evil" in the title. But the most frightening aspect of all is the Doctor's odd behavior throughout the whole ordeal; it gets to the point where his companion Jamie accuses him of being on the Daleks' side.
On a more uplifting note, this episode also marked the first appearance of Victoria Waterfield, whom the Doctor invited aboard the TARDIS after the death of her father during a Dalek battle. Only one episode survives intact, leaving the other six to be restored through animation.