4. The Fourth Doctor Chats With Davros (Genesis of the Daleks, 1975)
This remains one of the best stories of Doctor Who, both old and new. It features great questions of morality, some wonderful action scenes and introduces the character of Davros who, for better or worse, would cast a great shadow across the universe for decades to come. The best moment from this story, and a real high point for the series as a whole, is when the Doctor is being held prisoner by Davros, who makes the Doctor share the details of every Dalek defeat, hoping to prevent those defeats in the future. The two have a fascinating conversation where the Doctor learns just how very morally bankrupt Davros really is.