17. "Tomorrowland" (Season 4, Episode 13)
The milkshake that shook the world. The Season Four finale is easily the most controversial, divisive Mad Men twist ever, as Don takes the kids on a Disneyland vacation and returns engaged to the babysitter. It all comes down to one scene: When the Draper kids spill a milkshake at a diner, Don snarls, but Megan grabs napkins and says, "Don't get upset. It's just a milkshake." Don stares like she's from another planet. The kids look at each other in shock. You can see it on the faces of this miserable little family — everything's different now. A heartbreaker of a moment, and the milkshake epiphany makes Don suddenly decide to marry his foxy Montreal quicker-picker-upper. Another emotionally powerful scene: Don opens up to Sally about Dick, explaining, "That's my nickname sometimes." (And speaking of being a Dick — poor Faye.)
Pete Campbell, unlikely voice of social finesse: "You don't say 'congratulations' to the bride — you say 'best wishes.'" It's so depressing to agree with Pete Campbell about anything.
Best line: "Whatever could be on your mind?"—Joan Holloway Harris to Peggy Olson