2. Bad Wolf (Series One)
The one that set the precedent; the 'Bad Wolf' arc of Christopher Eccleston's single series was both exciting and mysterious, and subtle enough not to be at the detriment of standalone stories (unlike the River arc of series six). Many times the term was so embedded in the background of an episode that we barely even knew it was there; and it took the Doctor's sudden, concerned flashback in Boomtown to really point them out to us. This was skillfully done, because it is the very nature of the arc itself; a message littered through time, almost in the subconscious of the Doctor and Rose and therefore, vicariously, the viewer. It doesn't so much unfold as it does sit there gestating through the course of series one. It is graffitied onto the TARDIS by a troublesome kid, used as a name for a military chopper in 2012 Utah and is the name of a television company in the year 200,100. The point is, the people using those words have nothing in common with one another. This adds to the spookiness of the concept as Bad Wolf seems to be an omnipotent force, corrupting the conversations and activities of people throughout Earth's history (because Eccleston's Doctor is more or less Earth-bound for his entire run) in little ways, haunting the Doctor all the while. One of the best moments in the arc's run comes in the final episode; The Parting of the Ways. When the Doctor asks the Dalek Emperor what the words mean, he chillingly responds that he has no idea, they are not his doing. One more excellent twist just as we thought we had it figured out. And of course, the real culprit is just as spooky, incongruous and omniscient as the arc itself; Rose Tyler, possessed by the heart of the TARDIS and wielding the power of a God. Equipped with Billie Piper's natural, well-spoken accent and glowing white eyes, Bad Wolf becomes a self-fulfilling prophecy as the words can only come into being if they themselves lead Rose to creating them in the first place. What a marvelous end to a blistering debut series.