“The Naked Time” is a great bottle episode, if you know the phrase. There’s a significant crisis, the pacing is fast, there’s a good mix of humor and drama, it’s fun to see characters essentially being drunk for a protracted period of time, and they discover time travel at the end of the episode. Doesn’t get better than that! I always like Spock’s line near the beginning, too: “It could be some sort of space madness we’ve never heard of.” Yikes!
“Amok Time” is great drama and it shows a totally different and atypical side of Spock. Plus you get the first real introduction to Vulcan life.
“Space Seed”… I mean, it’s Ricardo Montalban. He’s masterful in that episode. And, of course, it sets the stage for Star Trek II, which is just a fantastic movie in its own right.
“A Piece of the Action” is just ridiculous fun.