What I admired the most about TOS was in its "pulp sci-fi" origins. The ability to recapture the feel of "Buck Rogers" and "Flash Gordon" with social commentary. I really got the impression that the writers of TOS cared about world history, politics, sociology, and anthropology, were well-versed in classical plays and literature, as well as areas of human psychology.
Not so with the TV writers employed on Modern Trek. The only time Modern Trek even came close to what I described above was during the 1st and 2nd seasons of TNG. But, afterwards, TNG gradually declined into formulaic TV writing where most of the scripts centered around your typical "conflict resolution" type stories. The same thing with VOY. These shows just did not have much to say. Writers like Brannon Braga simply did not care about world history, politics, sociology, human development, plays, or literature. Even "Seinfeld," a show about nothing had much more to say in the areas of social commentary and interpersonal human interactions/relationships than Modern Trek ever did.
Out of the 3, I suppose my vote would have to go to DS9. The fact that it featured characters like Kira Nerys who reminded you of the freedom fighters from Nazi occupied France, as well as races like the Cardassians as an allusion to Nazi Germany, and groups like the Dominion as a force much like "The Axis of Evil" from WWII (Germany, Italy, Japan) showed that the DS9 writers
at least cared about those same issues on a consistent basis as the TOS writers did. And episodes like "Past Tense, Parts I and II" showed that the writers acknowledged and cared about where our world's whole government, social, welfare system was headed, and "Far Beyond the Stars" admitted the racism of 20th century American society where writers were barred from portraying non-WASP characters in a positive light, except as negative, stereotypical caricatures of propaganda (Such as Ming the Merciless from "Flash Gordon"). Thank goodness DS9 had a producer who cared about quality and substance like Ira Behr who encouraged writers like Ronald D. Moore and Robert Wolfe not to hold back and write teleplays that pushed the envelope further.
I never saw much of that in TNG seasons 3-7 and VOY seasons 1-7. They truly looked and felt like products made and run by people who did not care in one way or another whether or not their shows had any bearing or origin to TOS, as long as they were making a hit and making a lot of money in the process.
At least on TOS, when the crew encountered a spacial anomaly, they at least tried to resolve their crises through human ingenuity and cooperation. Those scenes genuinely offered insights into the human condition within the confines of a futuristic setting. On a show like TNG and VOY, whenever the Enterprise-D or USS Voyager encountered a spacial anomaly, it was all about the technobabble solution that saves the day. And those episodes that deal with the human condition on TNG and VOY dealt with "side stories" about Data raising his cat, or Guinan's tennis wrist, or Troi's love of chocolate that had no bearing whatsoever in driving the main plot forward. What a complete waste of time and talent involved.
This is probably one of the main reasons why the TOS franchise had a wealth of material to draw from when they made their 6 feature films from 1979-1991. They had resonance with its audience. TNG, too easily caved into studio pressures when it came to producing the 4 TNG feature films. There was not one single fighter amongst the TNG production team to champion its cause, unlike TOS, because quite frankly when all things were said and done, TNG was "a hack show" that really had nothing much to say. Seriously, after 41 years of Star Trek, the only messages I seem to recall that have any practical worth and value in the real world are from TOS, early-TNG, and some episodes of DS9 during the course of its 7 seasons.
These are the reasons why I consider TOS to possess that "timeless quality" that all these other series that came afterwards never possessed in the first place.
