I guess I'm in the minority here where I thought DS9 was the most consistently good of the Star Trek series. I thought TNG had the best episodes, but watching some of the stuff from the early seasons made my eyes bleed.
DS9 basically broke all the rules of the first two Star Trek series. It allowed interpersonal conflicts between characters, allowed some gray and even some black to mix with the goody two shoes Federation, and allowed extended story arcs. In addition, because it stayed in one place, repercussions from earlier episodes can been seen later on and a whole host of interesting supporting characters could be developed. Damar, Dukat, Quark, Nog, Martok, and Winn are all examples of how characters outside of the main cast contributed to DS9.
One of my favorite moments was in the episode The Darkness and the Light, where Kira was confronted by one of her victims during her time in the Resistance. The man was a page for a general and was wounded in an attack on a base; he was essentially an innocent civilian. Instead of apologizing for actions during the war, she instead spit it right back in his face, essentially saying, "None of you belonged on our planet. In my mind, you were all legitimate targets." It strayed from the easy answer and instead stayed true to the character.