2. "Mirror, Mirror"
This episode also sparked the imagination of many viewers back then because it asked "what if" and was one of the first sci-fi episodes to suggest a parallel world within an established series. The opportunities are so numerous that Star Trek: Enterprise revisited this world "In a Mirror, Darkly," almost 40 years later! And who can forget Star Trek: Prodigy's "Cracked Mirror" episode in 2024?
Most fans are grateful for this episode because it's so undeniably cool. The characters are familiar, yet twisted. I especially love how ruthless Mirror Sulu is. And Kirk, Lt. Uhura, Scotty, and Dr. McCoy must do all they can to survive.
Nichelle Nichols is fabulous in this episode, and it's obvious she enjoyed having more to do than her usual, "Hailing frequencies open," dialog. It establishes her as a fine Starfleet officer who isn't afraid to reveal her fear over the unknown but faces it anyway, making her a role model for women everywhere in 1967.