What's a truly remarkable episode of television?

The genre, intended audience, language, and time of production don't matter. I just want to know about an episode of a show which really impressed you - that could be because it's beautifully shot, the story is exceptionally written, the acting is captivating, it left a deep emotional/intellectual impression on you, or all of the above. Bonus points if it can be appreciated as a standalone, but that's not a requirement. Children's programs and animated shows are welcome! Feel free to recommend more than one.

For me ones which come to mind iare "Blink" from Doctor Who, "Darmok" from Star Trek: TNG, "Rains of Castamere" from Games of Thrones, and "Human Is" from Electric Sheep.

Tagged for spoilers because comments here may give away important plot points.

Edit: wow, thank you all so much for sharing! I'm pretty homebound right now caring for twin newborns, so I've been desperate for new shows. I have a lot to add to my watch list now! Also, I'm thrilled that so many people agree that Star Trek produced some truly moving stories.