4. “The Tholian Web”
On a personal note, I think this might actually be the first piece of Star Trek that I ever saw. Written by Judy Burns and Chet Richards, with direction by Herb Wallerstein, “The Tholian Web” is an exciting episode that takes Captain Kirk almost entirely out of the action. This gives Spock and McCoy a chance to truly shine.
Granted, while Kirk is not present for the majority of the episode, he is still a major focus, as Spock strives to lead the crew of the Enterprise through multiple catastrophes. Not only is the captain seemingly dead, after all, the crew is also going mad from a phenomenon on the USS Defiant. Plus, they are dealing with the Tholians, who are not pleased at all with the Enterprise’s presence.
While Spock is shown in command on several other occasions, the appeal of “The Tholian Web” is that it is not actually about Spock being in command. Thus, he is shown as an effective commander leading the crew through a crisis. This is much more fun, in my opinion, than Spock’s leadership in an episode like “The Galileo Seven,” where Spock’s authority is incessantly questioned.
In addition to all of this, the Tholians are a simply iconic Star Trek species, despite almost never being seen. Their titular web is cool from a design perspective, and the mysteriousness of the Tholians helps to emphasize that not everything in space is known or understood.