Deep Space Nine seasons
With Star Trek TNG being an even bigger success than the Star Trek TOS films, the franchise wanted to expand further. Star Trek DS9 was a very controversial idea for a Star Trek show when it was created and did exceptionally poorly in the beginning. Among fan reactions were questions like, "you mean they're just going to sit there on a space station?" And, "no boldly going where no one has gone before?" Indeed, the wormhole introduced in the show's pilot offered a new (and different from TNG) frontier for DS9 to explore, but they still wouldn't be doing it very boldly. Essentially it would boil down to a few Runabouts going through the wormhole and a few alien visitors coming from the wormhole to the station. As a result of this meager premise, most people were still much more interested in the illustrious Enterprise-D cast, which manned the Federation's flagship rather than a newbie cast manning the unremarkable, almost penal assignment that was Deep Space Nine.
But all good things must come to an end and TNG came to an end. Star Trek Voyager took its place as the "boldly going" show, but like DS9 lacked the glamor that TNG seemed to have. Voyager was boldly going home with some exploration along the way. And as impressive as those Intrepid class ships were, they were not the Federation's flagships. So with the end of TNG, the writers focused heavily on developing the derivative shows more. At about this time DS9's Dominion arc began to emerge. The writers were beginning to realize that they needed something more than "the action will come to DS9 through the wormhole" as a plot device and that the show needed to establish and develop significant recurring villains. Doing this changed DS9 from a show of exploration into a show of galactic politics. One could also argue that Babylon 5's critical acclaim and cult following significantly influenced this decision as well.
So DS9 was gradually refined. The Defiant was introduced to the show to combat the Dominion threat in season 3 and Worf was added to the cast in season 4. The show slowly began to focus on the growing unstable political situation until finally at the end of season 5 all out war began when the Dominion attacked the station. The ensuing final two seasons of DS9 were easily the best storytelling ever to grace Star Trek to date. They were also arguably the best stories of the entire second generation of space opera. While there were a whole slew of memorable stand alone episodes such as The Visitor, Trials and Tribble-ations, For the Uniform, or Field of Fire the best episodes of DS9 were tied heavily into the Dominion war story arc. Among the most memorable were Call to Arms, The Sacrifice of Angels, The Changing Face of Evil, the finale What You Leave Behind, and the episode often cited as the Star Trek episode of all time In the Pale Moonlight.
Indeed, what made DS9 so great was that it was a prototype for a better kind of science fiction storytelling. The powers that be had the critical acclaim and money to produce a new kind of space opera show that they could experiment a little with. Watching DS9 from beginning to end allows you to see how they experimented and how their vision was so much more clearly refined by the time the show ended. My only complaint with DS9 was that it took them so long to find their way. Seasons 1 and 2 were quite rough, and even though the average episode quality rose sharply each season, the buildup to the Dominion war was frustratingly slow. In retrospect, seasons 1 and 2 (the exploration seasons) could have been compressed into a single season and seasons 3, 4, and 5 (the buildup to the Dominion war) could have been compressed into two seasons, leaving us with three seasons for the war instead of two, and the final season being dedicated to its aftermath and epilogue, instead of a single episode (the finale) being dedicated to that.
Hindsight is always 20/20 of course, and despite some stumbling, DS9 still ended up a stronger series than the other Star Treks in the end. It was a darker Star Trek with a war story. It was a deeper Star Trek which dedicated strings of episodes to thoroughly explore its plot threads. It was a fascinating Star Trek with riveting secondary characters like the mysterious and machiavellian Garak, the hilariously evil yet poignant Weyoun, the crazed and creepy Dukat, and the power-hungry female shape shifter. It was a show that took veteran Star Trek characters like O'Brien and Worf and integrated them seamlessly with an entirely new cast, making you love every single one of them. It had great actors, impressive visual effects (especially for the time), and innovative writing. Even shows in the following third generation of space opera such as Firefly and the Battlestar Galactica reboot were benchmarked against DS9's brilliance. Of all the Star Treks, this one stands out the most. It's both the most exciting and the most moving of them all. Truly the high point for all of Star Trek.