3. "Journey to Babel"
Often scoring high in "favorite episode" polls, there are plenty of reasons to be thankful for this entry. We meet Spock's parents, an array of alien ambassadors causing plenty of trouble for the crew, and one of my personal favorite antagonists, Thelev, who tries to assassinate Kirk and nearly succeeds!
William O'Connell (Thelev) steals every scene he's in, from his frowny resting face to his final moments on the bridge, which he plays like he's LARPing his character's own death. He's so dramatic that he gives William Shatner a run for the money.
We also see Spock gets his dry sassiness from his father. Sarek tells the Tellarite ambassador, Gav, that speaking to him seems unavoidable and then reminds the pushy being that personal space is a thing by shoving him away with hardly any effort at all, using his arms to block Gav's attack. It's a graceful move and somehow very Vulcan
Spock's mother, Amanda, is just as beautiful with her feathered outfits and motherly ways. I cannot leave out the ending of this either, where McCoy tells Spock to shut up before shushing Kirk, so he can tell us all that he finally got the last word. It's glorious!