I think that comparing DS9 and TNG is like comparing Kit Kats to Doritos. Personally, I like both. There are times when I'm in the mood for one, and times when I'm the mood for the other. I can understand people who only enjoy one or the other, or neither of them, for that matter. But arguments about which one is "better" falter, for me -- you can't compare them just because their package is the same color. They intended to do, and did, different things.
I feel that I can compare TOS, TNG, VOY, and ENT to each other, because those were all, to some extent, trying to do the same thing. Exploration, idea of the week, boldly going etc etc. In fact, I think that VOY and ENT got weakest when they went off-message and started going into story-arc mode -- but then, I stopped watching ENT in the second season, and I heard that the story arcs got stronger in Season Four.
I really liked having a Star Trek with a continuing storyline. I liked the political machinations -- but I wouldn't want every Star Trek to be like that (just like I don't want all Kit Kats to suddenly taste like Doritos). I always found the comparisons between B5 and DS9 to be more apt, since those two shows were trying to tell similar stories in a similar way. I thought that B5 had a stronger and more epic story, but I liked the political machinations in DS9 more, primarily because even with the wormhole aliens, I still felt like there was less Sudden Surprise Solution stuff in DS9 -- at least, once the war got rolling. In B5, you had Lorien pulling rabbits out of his hat and Sheridan doing big tricks with the telepaths, while DS9 didn't seem to add so many new cards to the table. (I liked both, mind you, but for different reasons.)
Which isn't really an answer. For my money:
TOS: Great for its time, a nice mixture of topical drama, cowboys-in-space, and current-for-the-time SF-idea exploration. I appreciate it, but watching it today, I find it fairly silly most of the time, because the groundbreaking stuff they did isn't groundbreaking any more. That's not a slam on TOS -- that's just just changing times.
TNG: Pretty much the same thing. Some episodes of TNG still hold up really well, but some episodes now seem dated and silly. At the time, I was entertained.
DS9: Enjoyed it, more so when the war story began. The war-arc seems to hold up better over time than the standalone TNG and TOS episodes, but we'll see if that stays true.
VOY: Stopped watching in Season... Five, I think, except for occasional episodes (I caught the finale). It seemed to pull its moral punches too much -- you'd have interesting moral ambiguity, and then one bunch of aliens would just decide to attack or commit genocide or something to make it clear who the bad guys were. (TOS and TNG might have done this too, but I was younger when I watched them and didn't catch it if they did it a lot.) And purely on a personal-taste note, I got tired of the Borg. I didn't need them in the series, although I appear to have been in the minority.
ENT: Watched it through the first season and into the second, then stopped. I never got past the feeling that it was pulling punches, trying to be clever, doing card tricks in the dark, something. It just seemed like a joke I wasn't in on, and I'd watched all the other Treks. I heard it picked up in the fourth season, but it was too late for me by then. I really liked the idea of the temporal cold war, and it just never clicked... for me. I know it clicked for other folks.
