The Original Series seasons and films
Star Trek TOS was the shining star of the first generation of space opera on television. No other show, not Battlestar Galactica, not Lost In Space, and not even the Star Wars films portrayed a more original, more hopeful vision for the future than the original Star Trek. Not just a vision of a better future and a united Earth, but a vision of a united Earth working together with alien civilizations. A United Federation of Planets largely free of racism, greed, bigotry, and corruption working together for mutual defense and the exploration of space. Star Trek TOS was a bigger show than anything else on TV. It did more with less money than anything else on TV. Star Trek TOS was a testament to how well a good story stands on its own without the aid of a high production budget. But despite its greatness it was still easily the worst of the Star Trek shows. There were a number of reasons for this.
Because Star Trek TOS was produced largely in the 1960s, a few of the poor sentiments of the day bled into the writing. For starters Star Trek TOS almost totally ignored the metric system in favor of English Imperial Units, one of many aesthetic details designed to downplay the idea behind the United Federation of Planets and connote instead that TOS was a show about Americans in space derived from an America not so dissimilar to the contemporary one. Additionally, Star Trek TOS had quite a bit of overt sexism inserted into the series. This was much to Gene Roddenberry's discontent, as the original TOS pilot The Cage presented a much more gender-equal setting. That pilot was rejected and Roddenberry was (perhaps in not so many words) asked to make the show more sexist. And so it was. Fortunately, later TOS productions largely removed the sexism during the 1980s. Star Trek eventually converted to the metric system as well.
TV networks' requirements went far beyond a few trivial details though. In order to boost ratings, Gene Roddenberry was forced to make many TOS episodes "less cerebral," which was thought would appeal to the average American more. Plots were simplified, fighting was frequent, and the science fiction was often dumbed down to the point of being devoid of any scientific accuracy whatsoever. TV networks did everything they could to make Star Trek an action show (despite its inadequate budget for this) rather than a profound story with a science fiction setting. Still though, every once in a while something profound bled through too. Episodes like All Our Yesterdays, The City on the Edge of Forever, The Day of the Dove, Errand of Mercy, Journey to Babel, Space Seed, and Balance of Terror among others are classics. But there were too few such episodes because Gene Roddenberry struggled against both TV network politics and the cultural norms of the times. In the beginning it looked bleak. TOS was canceled in February 1969 during its third season, just five months before the first lunar landing. Clumsily continuing the show via The Animated Series led to the brief revival fizzling out during its second season in 1974. Third time was the charm though. The TOS films produced starting in 1979 were nearly all huge successes.
Star Trek TOS was not a perfect series. It had more problems than any of its descendants. But in the end, better executed TV shows had less effect on people. Star Trek was the first space opera to attain mass appeal because it was the first to have a truly optimistic vision for humanity. It was way ahead of its time and it set the standard for future science fiction television. For this it has become perhaps the most famous space opera ever made. At the time it was being made, to paraphrase Star Trek I: The Motion Picture, the human adventure was just beginning. The legacy Star Trek TOS left behind was extraordinary. Anybody would be lucky to leave a legacy behind half as impressive as Gene Roddenberry's.