No show of the last fifty years or so, including Doctor Who, has so shaped what we think of as science fiction. Not even the Twilight Zone, which appears later on this list, had quite the same level of impact. That it did so while being, in general, a really good program is saying something. The best episodes, such as City of the Edge of Forever and Balance of Terror, still hold up very well; not just as great sci-fi stories, but as great stories period.