Another big strength of the show compared to other Trek is the supporting cast. Other Trek series may have a few returning characters but their appearances are often limited. Deep Space Nine has a much richer supporting crew that you'd almost think some of them would have been promoted to series regular with their name on the opening credits. Since it is on a space station, it allows for having a supporting cast of non-Starfleet officers whereas the other Trek series would have more supporting characters as Starfleet officers and on the ship since they were more likely to be re-encountered that way. Yes, there were other aliens and enemies they may re-encounter but those appearances were often quite limited. One of my favorite recurring characters in a Trek show was Shran the Andorrian in Enterprise and yet I think his number of appearances was around 9 or so over that whole run. It's the nature of a show with a mobile home base. The stationary base of Deep Space Nine allows for a growing supporting cast and good on the writers for realizing and taking advantage of it. (I was also bugged about a character seemingly being familiar to me on Deep Space Nine and I looked up the actor to find out it was Jeffrey Combs who plays Weyoun of the Vorta who plays Shran in Enterprise. That's why I recognized him. I was surprised to see just how many characters and how much of an impact Combs has had in various Trek series.)
Frankly, the supporting cast often seems to outshine the main cast and may be the biggest secret weapon of DS9's appeal. Dukat, Garak, Rom, Nog, Keiko, Martok, Weyoun, Leeta, Vedek Wynn, Vedek Bareil, and Ziyal all really help liven up the proceedings on the show. Not everyone is a winner though. Eddington took awhile to go anywhere. Cassidy Yates has seemed to fade away and I don't really miss her. The Grand Nagus kind of annoys me with his voice and seems at odds sometimes with this universe and mood of the show. Still, the majority are pretty great so good on the writers. A strong supporting cast like that can really do a lot for a show.
The other interesting thing about DS9 is how much it is interested in Trek history and its mythology. Not only does it show that actions have consequences from the story and situations the characters created for this series find themselves in but it's also willing to do this from storylines in TNG and even the Original Series (particularly the Mirror Universe for which it has been slowly crafting a neat little storyline of its own). If you like TNG then you really do owe it to yourself to watch this series as it is the best sequel to that series as it continues and deals with a lot of elements that show introduced. Voyager went to a completely different area of space with a different tone of survival in uncharted areas with very little crossover with the Trek shows that had come before (or were airing at the same time like DS9). Enterprise was set before all this and was about establishing some of the history that lead to all these other shows. With TNG, I know Roddenberry really didn't want to acknowledge the original series much in the beginning of its run but loosened up on that as the show established itself more as its own thing allowing the writers to delve into some of that past history.
DS9 seems to be the culmination of that as the show becomes a less focused on commenting about contemporary issues through a sci-fi setting or the crew encountering some sci-fi concept. It's focused more on the politics of this universe such as the diplomacy, maneuvering, balance of power, posturing and trying to keep the peace. It's trying to really broaden out the scope of what life might be like in this universe when you are stuck in one place and can't really escape a situation by piloting the starship to a new part of space. It's also interested in expanding what life is like for some of the species in it like Bajorans, Cardassians, Klingons, and Ferengi. That willingness to really get into what has come before really is what makes this show special.
If I had to compare it to another show, I think Person of Interest might be the closest match. It could touch on high sci-fi concepts like AI or social commentary like privacy while engaging in intrigue with different factions and shifting power yet it still did this by showing the growth of the main characters in it how their various actions and past led up to the situations they would face or find themselves in each new mission.
I'm not about to declare it the Best Trek series yet. We'll see how it all plays out and I still have some negatives in my mind about it that I haven't got into much here. However, I will say that after wishing someone would create a new Trek series for over a decade now, finally watching DS9 has felt like that wish being granted. This show has got me fired about Trek again that it's got me thinking about watching Voyager again and finally going through that series to get more of my Trek fix. It's got me thinking about rewatching TNG again since it's probably been 15 years since I've watched that series and I'd like to see those 6 and 7th seasons again to see how they were intermingling with DS9's early work. Recently, the trailer for Star Trek Discovery was released. I must say that I don't have much faith in that show. It looks like its more interested in copying the style and tone of the latest Trek films than in building on further what TNG/DS9 has created for a Trek Universe. Sure, the effects and visuals look great compared to what I'm seeing in DS9 but nothing about indicates the creative forces behind it understand the appeal of these earlier Trek series. Worse, they seem to be making the mistake for which Enterprise got a lot of hate by setting it in the past before TOS. Part of the reason why the Star Wars prequels were rejected by a lot of people is that what people wanted were more stories with the original cast of Luke, Leia, Han and Chewie. Its why the latest sequels have gotten such big buzz and people excited. (Although Force Awakens dropped the ball a lot on that expectation yet, even with that disappointment, the fact that Luke may play a big part of Episode 8 is still getting me excited for this next film). Guess I'll see how DS9 ends soon enough but I don't get why writers don't want to carry on from that point or even go a bit further in the future from it.