Here’s five of my favorites, especially because most of them provide some character lore/development that enhances watching the movies:
The Corbomite Maneuver - Kirk’s captaincy style
Amok Time - Spock lore
Journey to Babel - fun, ethical dilemma episode, introduces Sarek
Let That Be Your Last Battlefield - moral lesson episode, introduces the self-destruct, and I’ve found that modern viewers are fascinated by the fact that racism has become alien and confusing to future humans
The Galileo Seven - character drama, features Spock making survival choices that weigh the needs of the many against the needs of the few
Diversity that seems natural to the plot and diversity that seems to be the plot are treated very differently by different kinds of fans. The former seems to be well received even by people that aren’t huge fans of diversity. The latter seems to be grating on a large number of fans.
I actually really like social commentary, I don’t like tired, hamfisted, and insultingly performative puppet shows of social commentary. STD gave me vibes that a boardroom full of old white guys in expensive suits had a conference call with the writers that went something like “market research says progressive bullshit sells well to our target demographic, so I want 24 scripts about being ethnic or queer or something on my desk by morning. Make sure you really spell it out, too, the [slur]s are fucking stupid to get it otherwise.”