This is one of the most gripping hours of TV ever. I’ve seen all the TOS episodes dozens of times and this one stands out above them all.
Windom’s performance was amazing. The tension, sense of dread, the Enterprise crew’s efforts to figure out a way to combat the planet killer, Kirk’s stoicism in the face of potential imminent death- “Gentlemen, I suggest you beam me aboard”, the musical score, it all just works and holds up very well over 50 years later.