#13: Mirror, Mirror
In case you forgot: Kirk, McCoy, Scotty, and Uhura get transported into a parallel universe and an evil version of the Federation.
Just like pilots are fun to watch to see what the show could have been, "Mirror, Mirror" is fun to watch to see what the fictional Enterprise could have been. I like to think that everyone's evil counterpart is basically the same person, just in entirely different circumstances: Kirk might well resort to exploiting alien technology to climb ahead, Sulu might well be a spying rat and probable rapist (remember how he grabbed Uhura in "The Naked Time"?), but evil Spock remains dedicated to science and logic above all else, no matter what. (Quite the "Sieg Heil" the evil Empire Starfleet has going on.)
No one really holds the idiot ball in this episode, either, which is rare for a TOS episode. Kirk and the gang immediately suss to what's going on and make plans to deal with it accordingly. Likewise, the Federation crew aboard the good Enterprise do the sane thing and throw the entire maniacal landing party in the brig, where they belong. The only idiot ball gets handed off to McCoy, arguably, when he insists on trying to save evil Spock, but sometimes being a good person and being true to yourself is more important than anything else. (I also like to think that deep down, Bones is so fond of Spock and his green blood and pointy ears that he can't bear to let even an evil version of him die.)
But the whole romance with Marlena is just ehhh. I'm beginning to think that I'm just allergic to any romance in TOS at all, ever. I guess she serves a practical purpose in making Kirk aware of the Tantalus device, and in evening the odds in the fight between the good landing party and evil Sulu, but....eh.