This moment from “The Parting of the Ways” is the defining moment for the Doctor. He has the chance to destroy all of the Daleks, but it would also mean destroying all the humans on Earth at the time. The Emperor Dalek asks him, “What are you, coward or killer?” The Doctor responds the only way he should.
It was such a powerful moment to me that I actually paused the episode so I could sit there and ruminate for a bit. This man–the last of the Time Lords–passes on destroying those responsible for the deaths of his people because the human race would be caught in the crossfire. It meant even more to me after I finally got to “The Day of the Doctor.” Seeing what he went through as the War Doctor let us understand Nine a lot more, and his choice to be a coward is still the standard his later incarnations should live up to.
Yes, Ten, Eleven, and Twelve had great moments and probably even better quotes. Nine’s answer to the Emperor Dalek, though, is simply fantastic.