10. "Just Save Someone"
Pompeii is crumbling, the Doctor is walking away and Donna is crying, asking him to save someone. Sure, you might make excuses about how the Doctor knows he shouldn't interfere with history, but when does that ever stop him? This moment's one of Ten's most defining (yes, he's still Ten after 'The Day of the Doctor') because it typifies the darkness inside him; he wallows in his own self-pity of not being able to save anyone, and that's what nearly kills Caecilius and his family. This is a scene that places Donna squarely as the hero, and leaves the Doctor in much shadier territory; it's clear that he can't live without his companions, which makes the road to Time Lord Victorious all the sadder, and more ironic, considering that he began travelling alone because he was so upset with losing his friends. Ultimately though, it might prove to be a good job that he saved Caecilius. You know, given that Caecilius helped saved Gallifrey.