The embodiment of straightforward, downright entertaining TOS is supplied in "Mirror, Mirror," a high-concept outing in which Kirk, Bones, Scotty, and Uhura are beamed into a parallel universe where Starfleet is a barbaric organization in which murder is a common way of advancing in the ranks.
The mirror versions of the Enterprise characters are delightfully devious, especially a seriously deceitful Sulu. But what most shines in this episode is the brilliant way it envisions the mirror version of Spock. Aside from looking ultra-cool with a goatee, mirror-Spock's temperament is very much like that of the regular Spock. The way his intelligence and logic allow him to bring a calculated approach to volatile situations makes him every bit as fearsome as any other Starfleet barbarian ... yet he's a man who can be reasoned with when the situation warrants it.
Meanwhile, Kirk's approach to his end of the situation shows his ability to venture forward with prudent caution and a clever choice of words—although he gets to work his usual charms on the "captain's woman," Lt. Marlena Moreau (Barbara Luna). The ending highlights Kirk's adamant nature in trying to set things right by human standards—even when he's not in his own universe! Add that to a show where Scotty and Uhura are also effectively used, and you've got a classic.