CRM-114,
How do you feel DS9, a 90s sci-fi series, holds up today? I personally think all the post-TNG series hold up. I only think TOS looks and feels dated, even the remastered episodes. ST: ENTERPRISE probably holds up the best, due to the improved visuals.
Depends...
The f/x... these hold up fairly well, especially given they're model-based and not always CGI. Voyager's CGI may have been new at the time but sticks out now. DS9 could make paper airplanes moving along on fishing wires work but its strong visuals complement strong storylines that much more.
The storylines... DS9 hasn't held up - it's even futureproofed thanks to strong storylines, as well as melding ideas from TOS into the TNG timeline and showing there's more to the shiny happy Enterprise going on, of which they by and large nailed it. TNG's idealism is like a teddy bear, but DS9 is simply more compelling with its range of characters and storylines. And costumes, no Trek show ever looked so great in the sartorial department. VOY feels like a continuation of TNG, just after the TNG seasons 5-7 slump. I never really got into ENT and what I saw were probably good reasons why (sorry, not much into prequels, regardless of century...) TNG has dated in some ways, took a chance on how human mannerisms might evolve in 400 years and be more formal and Vulcan-like (which makes sense given the Vulcans' history with Earth), but the writing is crisp enough that a lot of it still works, especially seasons 2-4. Season 1 is still late-80s cheeseball and for all the complaining about today's shows being preachy and all that, try sitting through seasons 5-7. It's just as nails-on-chalkboard and brash. TOS - this goes into two categories: (1) the fact the 1960s were weird helps save the day when continuing goofy far-out ideas in space just did not work in later decades. Like space whales and incorporeal ball of light things. Then there's (2) where the attempts to get beyond sexism fails. TOS is in the 23rd century and multiple times we're told if not shown these things aren't issues. Yet women still leave the service to raise a family (no stay at home fathers?) or lines of dialogue that are casually sexist, even uttered by Spock on occasion (e.g. Amok Time, a few others). I cannot include "Turnabout Intruder" since it can't make up its mind if Lester is insane or if there's a legitimate issue implied. Probably the latter given circumstantial evidence but correlation is not always causation. Thankfully "Wolf in the Fold", an otherwise strong story, flagrantly uses sexism to gaudily and forcibly to make a plot point* - to the point that one can count it
twice, thus making up for the series finale.
* Scotty hits his head on a wall or something because the crewmember causing him to bump his head is female therefore Scotty now has resentment and hatred of all women, which even Dr McCoy hamfistedly shoehorns into the plot. In the very same scene that Scotty is drooling over all the women and how he's going to enjoy it on Argelius but at least his total resentment will be gone because he's had a few (and I don't mean "beers").
The acting... acting styles have changed over the decades as well. TOS was a product of its time, TNG shows a paradigm shift between "televised play" and "natural drama". The former may be dated, but it's not unwatchable just because it's televised play and doesn't feel like the real world. Being televised fiction set in the future, I wouldn't want it to mimic modern day water cooler lingo anyway.
TNG is starting to feel dated, TOS is really dated, but if you immerse yourself in them, this will go away more or less. DS9 and Voyager, don't seem dated as much to me. ENT, even less so. I do love these series. I wish a could take a forget pill so I could watch them all like new. I watched them to the point where I can't watch them anymore. I'd guess over 10 or more times all the way through...
Immersion helps, as well as reading up on the times (especially the 1960s), to understand the original perspectives and limitations of the era. Even, then rare episodes like "Charlie X" still hold up without needing to go that far since awkward teenagers are inevitable as potholes in April, mosquitoes in July, kids wanting to load up on sugar and get diabetes on the final day of October, and so on. I would almost recommend doing the opposite for TNG, given how many people had to get Roddenberry away from certain notions, like the one involving Troi and her having three-- so considering how crass "Justice"'s final rewrite was, that still can't hold a candle compared to some pre-production ideas that were not carried into the show,
thankfully...