8. The Doctor Leaving Susan Behind (The Dalek Invasion of Earth, 1964)
http://www.youtube.com/watch?v=iHPVfkm58sw Susan was the Doctors granddaughter (wrap your minds around that, fans who have only seen the new series). She was a sixteen-year-old girl (apparently), who had travelled with him since theyd left Gallifrey. They clearly, deeply loved each other. But after theyd thwarted the Daleks plan to hollow out the Earth and turn it into a giant spaceship (look, just go with it, ok?), the Doctor came to realize that Susan had fallen in love with a resistance fighter shed met. After some consideration, he decided to leave her behind, thus making Susan the first companion to leave the TARDIS. It was a beautiful and touching scene, even now is kind of the gold standard for a scene where a companion leaves. One final note, for the curious whod like to learn what happened with Susan after the Doctor left her, check out the Big Finish audio story,
An Earthly Child, where the Eighth Doctor is reunited with Susan and meets her son, his great-grandson, Alex.