DS9 is far and away the best series of all of them, including TOS.
DS9 is much more detailed and must be watched in order, especially the later seasons. In the 90s in syndication, it came on after TNG on my local station. The other series, especially TOS and TNG were one episode stories, it did not much matter if you missed an episode. On DS9, there were long story arcs, and a lot of backstory
Obsidian Order, Central Command, The Prophets, Pah Wraiths, The Dominion. When I first watched it, the Cardassians were talking about Terek Nor, and I thought, "WTF is a Terek Nor?" As a poor, impoverished guy watching this series from an ancient black and white T.V. late at night after a tiring evening of employment, keeping up with all the stories, backstories, and races sometimes got a bit confusing.
What I did not like about DS9:
1.) The Dr. Bashir episodes, especially the episodes with the genetically modified people, the crazy guy with the mustache, the "vamp" woman, the slightly retarded old man, and the catatonic young girl. Barf. Move on.
2.) I really disliked any holodeck episodes from any of the series, and Dick Fontaine was no different.
3.) The Bajorans storylines. I think the Bajoran vs. Cardassian storylines gave off the wrong message, that is "beautiful is good" while "ugly is bad."
4.) The "Female Shape Shifter". Ever ask yourself, does the Founders have different sexes? Seems strange that the leader of the Dominion would take the form of a matronly older woman with large breasts, who would independently take the shape of what Odo looked like.
5.) Sisko and his "I'm black and proud routine." Isn't the idea of Star Trek is to show all races/colors of people to not matter? He wont go into Vic's holosuite because back in that time setting that blacks were discriminated against. We get it Sisko, you are black. So is Worf, but he's not wearing a NOI fez hat.
6.) The love stories. This is some thing I dislike from all the Trek series. (Or honestly from all series). In science fiction, I want to see space, space ships, action, weird aliens, and phaser fire. The Kira/Odo relationship was odd to me. Also, you never saw them in bed together. How does a woman make love to a shapeshifter? Does Odo shape a penis a certain size? Since Kira goes both ways (she's a closet bisexual), does Odo shape himself into Jadzia? How does that work?
7.) Keiko O'Brien. Any storyline with her, I skip. I really dislike the episode where she comes back to the staion and was possessed by an evil alien. Funny though, I am a westerner, married to a Chinese woman in the Zhongguo Star System.
8.) The Bajoran religion and those "prophets". What are those wormhole aliens saying? What do they want? Why don't they just come out from behind the shadows? Why take Sisko in the end, away from his wife, baby and son? That to me was very sad.
9.) The Breen! The dumbest evil aliens in the galaxy! Take off those helmets, you ugly bastards!