What's the single best episode of any genre TV show ever?

Science fiction and fantasy can change your life — and sometimes, the most powerful stories are the ones which change the lives of characters we've gotten to know. Sometimes one single episode of a TV show can make an indelible impression. But what's the best episode of science fiction or fantasy television?

Please include a clip or still image of your chosen episode. Also, please list the name of the show, the episode title, the year it aired, and why it's better than every single other episode of television ever.