I've held for a long time that Star Trek's "City on the Edge of Forever" would always be my answer to that question. After all it was the first time that an episode of sci-fi television was not only celebrated as "good for a sci-fi show" but just flat out great television regardless of genre.
But I've seen better tv episodes in fantasy/sci-fi. My favorite would be either the Carnivale 2-parter "Babylon" and "Pick a Number" or Twilight Zone's "A Game of Pool" (the original, by George Johnson.) As for Firefly (as some have mentioned) I fully believe that HAD Firefly been allowed to continue, an episode of Firefly would have eventually topped this list for me.
But I've seen better tv episodes in fantasy/sci-fi. My favorite would be either the Carnivale 2-parter "Babylon" and "Pick a Number" or Twilight Zone's "A Game of Pool" (the original, by George Johnson.) As for Firefly (as some have mentioned) I fully believe that HAD Firefly been allowed to continue, an episode of Firefly would have eventually topped this list for me.