4. "The God Complex"
This episode is so frightening I almost couldn't finish watching it. "The God Complex" is a psychological thriller unlike any other in the Doctor Who canon. A hotel which constantly rearranges its layout, rooms that lure you to experience your deepest, darkest fear, and a beast who brainwashes his prey to chant "praise him"? I'm quaking just writing that sentence. What's particularly brilliant about this episode is the way it approaches the concept of faith. Since the Doctor is himself a (Time) Lord, there is little room for discussion of other deities. The Doctor as a object of worship is often implied but never openly explored in the show, so acknowledging that his role as a demigod is fascinating. "The God Complex" also finally brings to light the concept that had first laid roots in "The Waters of Mars" and continued to grow throughout the next series and half: the Doctor as the infallible genius. Supported by three-dimensional secondary characters, this episode is a stand-out in every way.