Even after 50+ years (30+ for me personally) TOS remains the pinnacle of Trek for me. All the other series have highlights and their merits for sure, but there is just a visceral love of the series through and through no matter the episode. I attribute this to obviously the performances, ideas, characters, and writing. Though I also believe the color pallette and especially the music appeal to the eyes and to the ears more so than the modern iterations. Exposure since a young age from beta TV recordings to Blu Ray quality have reinforced this feeling over the years. At 34 years of age, I wonder how many others my age feel the same, what with all the alternative forms of entertainment options out there.
The color palette is very much 1960s due to the rise of color television becoming standard in 1966, though a handful of shows had used color before then. I personally adore what the designers did with color and lighting, but few shows are going to be made today with that style. Even TNG didn't try to recreate the colorful look beyond the costuming. DS9 comes the closest - and is more refined, at least with sets. Many alien species have luxurious, detailed, and colorful costumes of the sort that look flat on standard-definition but would be eye-poppingly gorgeous given the larger gamut that Blu-Ray can store. The only other era that wanted to be as rich with costuming would be TOS, but they often had no budget.
TOS also had a limited amount of music compared to TNG-onward. Incidental music is best when used sparingly, to let the actors really steal the show, but it has its place. I'll admit, Ron Jones' contribution helped elevate a number of early episodes and he shouldn't have been fired. Shame he never got to do DS9...
TOS also dared to show
epic battles on what amounted to a shoestring budget. The CGI folk who created new scenes stayed true to the stories rather than throwing in any old nonsense the way Star Wars and Red Dwarf had. The result is TOS-R feels even more luxurious, "The Enterprise Incident", "Space Seed", and "The Doomsday Machine" being some excellent examples of little nuances you just
know the original makers would have wanted to do. Indeed, the only TOS episode that I stick to the original effects with is "The Immunity Syndrome" - the remastered edition did a great job but something about the oil/water and lavalamp effects feel just slightly more tangible, even of the remastered edition took into place the opaque nature of the fluid mass, which II also appreciate.
Plotwise, a lot of TOS holds up. some doesn't. But DS9 won me over with taking the same premise as TOS (a final frontier, mystery, character conflicts, human condition, etc) and expanding on it, even with a space station - though the Defiant was the perfect addition to the show since they then get the best of both worlds (snarf)...
I can imagine TOS doing a "Section 31" sort of arc. Not so for the others. TNG doesn't get that deep and the others might be out of place and ENT was too busy with its own continuity.
And, indeed, would Kirk do what Sisko did to get a traitor like Eddington? I'd be borderline but leaning toward "Yes". I can't fathom any other captain doing that, much less succeeding.
Sisko rules.
As much as I love TNG and Voyager, DS9 and TOS are tops and of those two DS9 ultimately wins as it does everything TOS did but took it on a bigger scale, without losing itself.