1. "Planet Of The Ood"
If you have ever watched "Planet of the Ood" and not cried or at the very least fought back a lump in your throat then you need to check yourself for a soul. The introduction of the Ood in previous episodes of Doctor Who suggests that they might be a more complex alien race than simply born servants, and "Planet of the Ood" finally reveals them to be bred slaves who are oppressed across the galaxy. The Ood are a beautiful, gentle race, born with their brains in their hands, and their song of unity is both sad and uplifting. This episode features some heavy social commentary, drawing parallels to marginalization and enslavement of people throughout history and to present day. There is a telling moment when the Doctor implores Solana, the employee of the Ood's oppressors, to join him and Donna to free the Ood. She hesitates, considering the Doctor's statement that she knows what's happening is wrong, and turns them over to the authorities. Solana is clearly conflicted in her moral judgment, giving her character a profound level of depth. However, "Planet of the Ood" is the perhaps the most underrated episode of Doctor Who not only for it's complex and deeply emotional plotline, but also for it's vital role in foreshadowing the 10th Doctor's epic conclusion by introducing us to the DoctorDonna. What seems like a throwaway name comes to have immense significance later; these little tidbits ensure that the story not only has weight in its immediate viewing, but also carries a heavy impact throughout the larger story arc.