7. The Sacrifice Of Captain Jack
This may be controversial seeing as it's never officially been confirmed in the Doctor Who history that Captain Jack and the Face of Boe were indeed one and the same. Assuming that Jack's revelation at the end of Series 3 was as it seemed, though, it's wholly possible to suggest that Captain Jack Harkness, from the moment after his first goodbye to the Doctor, gave everything he had to save as many people as he could. It was as the Face of Boe, however, that Jack Harkness literally gave his all. Whilst visiting New New York in the Series 3 episode Gridlock, it's revealed that the entire surface population of New New York had been wiped out by a virus and in order to keep the remaining population alive, the Face of Boe had attached himself to the system in order to keep the motorway running. In the final moments of the episode, the former Jack Harkness gave the remainder of his life force to open the ceiling of the motorway and release those trapped inside. This moment of self-sacrifice leads to death of a being that has lived for, perhaps, billions of years and was thought to be immortal. To give up something he'd had for so long took a huge sacrifice proved that Captain Jack had grown into the hero that he never wanted to be.