While I love the series, I think if I could only own one episode, it would be Emissary. I think the episode had everything that made DS9 so great, and I could show it to someone who might not have seen Star Trek before. You had probably Star Trek's best Captain in Picard (I know it's debatable but he was my favorite), A well written character drama piece with Sisko about a single father unable to let go, and it introduced the elements that made DS9 great to begin with. That, and it was 90 minutes so you get an added bonus.