First I'm 46 and watched TOS when I was young and it's still a reference for me. Then I liked TNG as it remained strong with the stories and characters, and maintained the "feel" of Star Trek with the values of self discovery, friendship, sacrifice and cooperation with unknown people. Voyager was a sad joke with annoying personnel and incoherent stories. Didn't have the courage to watch Enterprise as Scott Bakula lacks charisma so much. The new movies may be good for teenagers liking bland characters and action. So back to DS9. As usual the first seasons are there to introduce the characters and the specifics of the setting. This is done so well, with Kira and Garak introducing the Bajoran and Cardassian cultures and antagonisms (and strong recurring secondary characters such as Opaka, Winn, Dukat, Damar), Quark for the Ferengi, annoying at first, but growing as interesting and fun (and Rom, Nog), Dax with the Trill, Sisko, Bashir, O'Brien for the humans, Odo for the changeling. DS9 developed these characters, their motivations, secrets, changes masterfully. Take O'Brien (previously seen in TNG), he has some of the darkest episodes (Hard Time) and even develops a family. Dax has 3 different characters. Garak is both a tailor and an espionnage agent. And then the Dominion conflict brings a continuing story, with some of the characters having their feelings change more or less. That's where the series shows it is well written and built, as the war is the center, but there are side stories too that are great.