It's a great question, with answers that could change every day.
If I had to pick just one, one episode that I would hand a new viewer, one episode that I think really sums up Star Trek, it would probably be "Duet".
In some respects, it's not a typical episode. There's little to no technology involved, beyond the idea of people travel from one place an go to another. The surgery that Aamin goes through is not terribly beyond modern day.
But in the ideal of Star Trek as a mirror held up to humanity, oh does this excel.
It works best if you’ve seen the episodes before, but even within its runtime, you can see Kira transform. She starts off as someone who wouldn’t argue with the sentiment that he deserves his fate, just for being at Galitep, and even if she wouldn’t want to admit it, I believe just for being Cardassian. She might not be like the drunk at the end, she wouldn’t go stabbing random cardassians, but she’d shed no tears for any found dead.
By the end, she’s the ideal of what we want to be. Someone who has found peace and forgiveness, someone who has grown.
And the acting! Sir Patrick Stewart is arguably the best of any of the main actors in Star Trek ever, but in these few minutes and scenes, Harris Yuulin tears through scenery and demands your rapt attention.
He’s able to manipulate you through hating him to pity and then grief - even when you know the real story.
Then there’s the real world echoes. Even as the real world gets ‘darker’ and there is an uncomfortable rise is sentiments that echo Nazis, its very easy to be Kira. To think, who could ever go along with that? How could you not see the evil? And if you did, how could you not fight against it?
MARRITZA: That's not true. I am alive. I'll always be alive. It's Marritza who's dead. Marritza, who was only good for cowering under his bunk and weeping like a woman. Who, every night, covered his ears because he couldn't bear to hear the screaming for mercy of the Bajorans.
I covered my ears every night. I couldn't bear to hear those horrible screams. You have no idea what it's like to be a coward, to see these horrors and do nothing. Marritza's dead. He deserves to be dead.
With these lines, I cannot help but picture some poor German soldier. Perhaps growing up brainwashed in the Hitler youth, perhaps drafted, perhaps joining looking for a warm meal. But then being hopelessly over his head.
In these lines, I find empathy and compassion. I can imagine being that young file clerk, trapped in a hell I cannot comprehend. Too scared to do the right thing. Too weak to grab a phaser and rescue everyone I can.
And at the end of it, I find forgiveness for him. We can all hope to be better. We all should be better. But too often, we are too human.
If Star Trek at its core is exulting us to be that better person, then it’s also a cautionary tale of what we all can be if we fail to live up to those standards, and a plea for compassion.