8. The Face of Boe
The Face of Boe has been an oddity and curiosity since his (its?) first appearance in The End of the World. I think that makes him the second-longest-running mystery in the new series, the first being Doctor Who? He is the oldest living creature in the Isop Galaxy. He was somehow pregnant at one point. He went out in a blaze of glory by sacrificing himself to save a people. We get one hint about who he is in Last of the Time Lords, when Captain Jack Harkness tells the story of being recruited to the Time Agency. He calls himself the Face of Boe (hereafter called the FOB for ease). Jack, the Tenth Doctor and Russell T. Davies all state in a Torchwood featurette that they believe Jack to be the FOB. Then Davies retracts a few years later by saying that Jack might not live long enough to be the FOB. When the person who came up with the character blusters that the idea is just conjecture, he's worth listening to. That said, I would like to know who he is if not Jack Harkness. He has millions of years of possible backstory. I would like to know when he gave up on the rest of his human bits. I want to know if he still had a thriving love life until he was nothing more than a well-tended skull.