^Well, it's not as if "The Doomsday Machine" is a particularly deep episode. It's basically TOS's special-effects extravaganza, a story driven mainly by action and spectacle over story. If you ask me, the only thing that makes it work is Sol Kaplan's fantastic musical score. If it had been tracked with stock music instead, I don't think it would have anywhere near the same reputation.
Or maybe people like some action and spectacle.
As for the story - well - the episode was nominated for a Hugo award. However, I won't speak to the criteria used for the nomination. Fans also seem to really like Balance of Terror
, too, a lot
. The music of The Doomsday Machine
is memorable, but that's not why I like the episode so much. I personally like it just because it's a space battle, and it was pulled off without getting too cheesy. It's a nice change of pace.
Fighting a giant robot in space is also an intriguing concept. Kirk's hypothesis of the backstory is fascinating, beyond the subtext of Armageddon, and raises some interesting unanswered questions (that I prefer to remain unanswered) with respect to exactly who was fighting the war.
Well, "The Doomsday Machine" was an science fiction story with a strong science fiction premise, written by a remarkably good science fiction writer. Most of the worthwhile TOS episodes were, to some strong degree, representative of so-called "Golden Age" sf and pulp sf of the 1950s and most were written by exceptional writers of the prose form of the genre - Ellison, Sturgeon, Bixby, Bloch, Sohl...
By the middle of the second year, though, the producers started settling for "Star Trek stories," a pretty hermetic and self-referential subgenre all its own which came to dominate the modern Trek franchise.