18. The Horror Of Fang Rock (Fourth Doctor)
Agatha Christie + Hammer Horror movies + Doctor Who. While this may sound similar to my description for The Brain of Morbius, I assure you this story is very much different. The Doctor, his companion Leela, and six others are trapped inside a lighthouse during a terrible storm, but one of the people is killing the others, leading everyone to try to discover the identity of the murderer. Eventually the Doctor is able to deduce that the murderer is in fact an alien capable of shape-shifting, leading to the investigation being much more difficult. We get some first-class acting from everyone involved, and some of the most intense scenes from this era of Dr. Who.
17. The Waters Of Mars (Tenth Doctor)
The next to last story to feature David Tennant as the tenth Doctor, The Waters Of Mars presents a brilliant look at the consequences of decisions the Doctor makes, and the battle of his conscience in the process of making said decisions. When the Doctor lands on Mars in 2059, he comes across a base set up by humans on the red planet. He quickly realizes this as a doomed expedition, that that very day will result in an unknown catastrophe that kills all inside. So he must choose between doing what he feels is right - rescuing them - or doing what he
knows is right - letting history run its course, leading to their deaths. Another bonus for the episode is the portrayal of the humans infected by a mysterious water-based creature. All this wrapped up into one package led to this story winning a Hugo Award for Best Dramatic Presentation... one of the highest honors for a work of science fiction.