21. "The Beautiful Girls" (Season 4, Episode 9)
Sally runs away to visit Don at the office, which means awkward improvisations for the ladies in his life — Peggy, Joan, Megan and (especially) Dr. Faye. Peggy tangles with Abe, the most unbearable of her never-remotely-bearable boyfriends, with help from her lesbian pal Zosia Mamet. And there's a royal farewell for the Queen of Perversions. Bonus points for some Austin Powers-style bedroom banter: "Is it broken?" "The lamp?"
Best line: "She was born in 1898 in a barn. She died on the 37th floor of a skyscraper. She's an astronaut."—Bert Cooper