Just MHO.
TOS was extremely groundbreaking for a few reasons:
Earth as a socialist utopia
Japanese, Russian, and female/African-American bridge officers
Interracial kiss
The series was very bold and forward-thinking.
TNG started out pretty lost and campy, but quickly found its way to some very solid character development and stories. They did the best job of the spinoffs as presenting space as a mysterious, large, fascinating, and dangerous place.
The Borg are probably the best science-fiction enemy ever to be created. Definitely up there with Skynet/Terminators, Alien, Predator. The only thing worse than an ultimate evil is ultimate indifference.
DS9 definitely had the best depth of character development of all of the series, examining many TNG races that were previously only scratched on the surface (Cardassians, Ferengi, Bajorans, Klingons, Mirror Universe,). We also got to know Chief O'Brien and Worf a lot better, particularly O'Brien's experience as a war veteran and resulting PSD/racism toward Cardassians.
As far as Trek girls go, I have only one thing to say: Chase Masterson leaning over a Dabo table.
DS9 was a look at a not-so-friendly universe where very bad things can happen any day of the week. It's a different look at the universe where wholesale exploitation, gambling, vice, spying, plotting, conquest, conflict, slavery, terrorism, racism, betrayal, fanaticism, genocide, war crimes and all of the ugly facts are alive and well in a place that hangs on the edge of the known universe. The aforementioned "A Walk in the Pale Moonlight" and the episode where Ezri Dax channels a repressed serial killer identity to catch a Vulcan sniper serial killer aboard the station are some of my favorites.
If you gave up on DS9 early, just start watching after the introduction of the Starfleet warship Defiant, and I bet you will be quite taken with the development of things. DS9 had to break free of the "old ways" and come into it's own.
I watched the entire Voyager series, but it was more than a little too goofy for my taste. I'm quite surprised that with the struggle to maintain viewers, that they decided to use pretty much this same formula for Enterprise. Even the Borg were cut down a notch in their formidability/fear factor. I was NOT a fan of that "fluidic space" story line at all. I found a lot of the characters watered-down or just annoying as well. Where deeper development on DS9 was revealing and interesting; Voyager's was usually lame and forgettable. Tuvok the Vulcan and B'Elanna the Klingon were headliners of a list I call "100 Thing I never Wanted to Know About Vulcans and Klingons". Chakotay the Native American? I always groaned when I realized I was about to watch a "Chakotay Episode". Nothing was worse than Tom Paris, however. Started of as a complete dingus, and acted like a teenager for the rest of the series. Neelix struggled to find a place and purpose on the ship. I think viewers also wondered what the heck he was doing on the ship, and what his purpose was. No big shock when they did away with that character. Kes? Same thing. Jeri Ryan as Seven-Of-Nine was a huge step in the right direction.
I give major credit to Kate Mulgrew as Captain Janeway. She and Seven were much needed breaths of fresh air on the warp-powered Island of Misfit Toys.
Enterprise? Wow. What to say about that. Nothing about that whole show made any sense at all. I wouldn't trust an undisciplined idiot like Captain Archer to run a fishing charter, let alone a space ship. Coming from a military family, I found Archer completely unbelievable as a Captain. He was more like a manager of a Applebee's than a starship captain. His "Darn it! This is really hard and I don't know what's going on!" approach to captaincy was a major letdown. And since when did the Vulcans become such a bunch of apathetic a-holes anyway? Rest of crew, besides Trip Tucker was seriously forgettable.
Xindi War was really good. Archer finally turning into a grown-up was nice, but too little, too late.
New Trek movies? Started good, but moved in the wrong direction fast. Beyond is kind of bringing it back on the rails, but they really have to do a much better job on the next film. Karl Urban is great as McCoy. Zachary Quinto does a great job as a young Spock. The "alternative lifestyle" Sulu? Why? Even George Takei (who is gay) thought that was a bad idea.
I'm really hoping that they get their act together for the next movie. If they can, maybe we might even see a return of Trek on TV.
