I'd say that Back to Reality is the single best episode of Dwarf, and possibly the single best individual episode of any show ever.
However, it is the culmination of 5 series of shows, and on it's own the viewer wouldn't understand most of the jokes.
If I had to pick out one 30-60 minute slot to show the greatness of science fiction, I would choose an episode of The Twilight Zone (original b&w series). I think that whilst some of the episodes that have been mentioned are both great and memorable, they perhaps fall into the 'fantasy' rather than 'sci-fi' category.
I think my choice would be The Monsters Are Due on Maple Street, which builds tension throughout the show to a satisfactory and unexpected conclusion. Although the majority episodes of Black Mirror would serve just as well.