7. The Doctor's Daughter
Sticking with Ten now, and that time he met his daughter... who would go on to become his wife... who's actually the daughter of himself. Yeah, this episode is weird when you get meta with it.
The Doctor's Daughter takes place on a planet where human beings use cloning to provide soldiers for their war against the Hath. When the Doctor is cloned upon arrival, we get Jenny, his "daughter", who is played by Georgia Moffett - Fifth Doctor Peter Davison's real-life daughter, and David Tennant's real-life wife. Again... meta.
After growing attached to Jenny across the episode, the Doctor is devastated when she takes a bullet for him and dies from her injuries. However, after he leaves the planet, Jenny is brought back to life via the mystical power of the Source.
Fans assumed that Jenny was a one-off character; nothing more than a plot device to teach the Doctor some lessons about parenthood. Seeing her get revived and then jet off into space on her own adventures was pretty surprising, and left viewers hoping for further interactions with Jenny in the future.
It's just a shame that we haven't seen her on TV since this episode aired in 2008.