This is the last of my skipped episodes. Of course, if you've been playing along at home, by the end of this post you'll know what my favorite Star Trek: TOS episode is just by process of elimination, but whatever! It's still exciting!
Where it should have been: #8, making it one of the best TOS episodes of all time.
In case you forgot: While answering a distress signal from the USS Constellation, the Enterprise encounters a planet-eating "doomsday machine." While Kirk is stuck on the broken-down Constellation, Decker takes over the Enterprise and goes full Ahab. Kirk manages to destroy the killer tube sock and returns to the Enterprise.
The good: "The Doomsday Machine" is a great sci-fi premise, and the story that plays out is well-paced and engaging. We've had Federation blowhards intent on usurping Kirk's command before (and we will again), but this episode is the only one where said blowhard has an entirely believable (if insane and misguided) motivation in doing so. After all, Decker had to listen to the cries of his dying crew; by trying to do the honorable thing and go down with his ship, he sent more than 400 people to their deaths. You get why he wants to destroy this thing.
The bad: Decker's obsession is great, but the way that Spock insists on tolerating it is incredibly frustrating. Refusing to submit to a psychological evaluation would be more than enough grounds to declare him unfit for duty. It comes across as an easy way to ramp up the tension.