I watched Spock's Brain recently. What struck me about the episode was they it showed the entire cast working together. The whole scene on the bridge involving Kirk, Sulu, Chekov and Uhura as they discussed where Spock's brain could have been taken, I felt really gave a good glimpse at what Star Trek might have been like if it had been more of an ensemble show like the succeeding series's. I can see why this episode was chosen as the third season opener.