7. Army of Ghosts/Doomsday (Series 2, Episode 12 & 13 - 2006)
Weve seen how big of a deal it is when The Doctor takes his leave, but this two parter was the first time we ever saw a New Who companion leave. As we learn in this story, Companions tend to leave in one of two most likely manners: they die (which is really more of a Classic Who occurrence) , or they endure some sort of event that basically closes them off to The Doctors life. (Though Martha Jones is an exception, as she walked away on her own free will. Also, Russell T. Davies could never resist bringing EVERY SINGLE ONE of his departed companions back for some sort of reason or another. So while you may be upset with this episode at the time, youll be ok.) But even bigger than the unfortunate departure of Rose Tyler is the villainous showdown that Classic Who fanboys had been waiting for all of their yearsCybermen vs. The Daleks. RTD really breaks out the big guns in this one: invasion armies, inter-dimensional voids, background time radiation particles that can be picked up with 3D glasses all of it is here. Ultimately though, this is the story of The Doctor losing the woman he loves to a boundary he cannot cross. Separated, they will never be able to see or love each other againat least until Davies completely undermines himself with the clusterfluff that is The Stolen Earth/Journeys End. Suffice it to say that when Davies wants to write an ending, he can write an endingand this is the proof.