Devil in the Dark's one of my favorites too, despite the monster looking like a giant glob of pepperoni and cheese. still, considering the effects quality at the time, that's okay. I think it was a thoughtful, layered episode; those are the best of the old series, where it's dealing with social issues (like two sides to every story sometimes leading to conflict, and how we can negotiate and serve both sides if we care to listen), or just stories that focus on what it means to be human.
Then on occasion, we get something crazy like the Yanks vs <whatever they were>, and a US flag with Kirk waxing patriotically poetic... yjkes. But even that is entertaining in its own way. When I was older, I liked more of the drama of TNG, and then after I had been been invested in TNG for some time, it started to get bland, boring, predictable, kind of toothless; and I missed TOS. Don't get me wrong, Patrick Stewart is an excellent actor, and there were some nice dramatic episodes (like The Most Toys with Saul Rubinek, or The Inner Light), but there was a rawness to TOS that is still appealing.
I read all the James Blish adaptations of TOS when I was probably twelve or so, and years later, that one scene where a female Yeoman is turned into a geometric shape and crushed to death stays with me. That was horrific.