6. "Nixon vs. Kennedy" (Season 1, Episode 12)
The first episode I saw, and the one that got me obsessed. Don's secret past comes to light, leading to a showdown in the office. Rachel Menken, who for some of us will always be the Don paramour who got away, asks him the question nobody else knows him well enough to ask: "What are you, 15 years old?" Don relives his harrowing war memories from Korea. And he meets the woman on the train, probably the first sympathetic female voice he ever heard, changing his life and giving him a lifelong thing for long-necked brunettes. Practically all his lovers resemble her somehow — with the notable exception of his wife Betty.
Best line: "You've got your whole life ahead of you. Forget that boy in the box. Look at you. Let me buy a soldier a drink."—The Train Woman