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Best: The mystery of Bob Benson
Key episodes: "The Doorway" (601/602); "For Immediate Release" (606); "Man with a Plan" (607); "The Better Half" (609); "Favors" (611); "The Quality of Mercy" (612); "In Care Of" (613)
James Wolk's guest arc as Bob Benson is sure to be remembered as one of Season 6's signature storylines, and the wild speculation his mysterious background ignited among fans proved the show still captures the pop culture zeitgeist. Some thought Bob was a secret psycho killer, while others were convinced he must be the secret son Don never knew he had. But the truth turned out to be a fusion of two other theories: Bob is a closeted gay man (with a crush on Pete Campbell, no less), who wormed his way into SCDP with a fake name and background a la Don Draper.
Pete deciding to keep Bob's secrets quiet (instead of trying to rat him out like Pete did with Don) was one of the best plot twists of the year.