It is hard to compare TNG, DS9 or Voyager, they all excelled in their respective domains (alpha, beta/gamma and delta quadrants :)). I measure the series on two scales, the sit down for an hour and watch rating and the suck me into a slowly developing storyline rating. On the first rating an episode is like a shot of espresso, a shot of trek to keep you going. The second rating is something you savour, new tastes and aromas slowly revealing themselves, we get a deeper look at the Trek universe and its inhabitants.
TNG had the best balance on these two ratings. Sure there were stories that started and concluded within the hour but it had several excellent longer storylines (Q, Worf's relationship with the Klingon Empire, Troi's mother, Troi and Riker, Data discovering himself).
DS9 is so rich and complex that it is almost incomparable with TNG. I loved the way the characters developed, particularly Bashir (who for me was the dark horse of the series). It was clear the producers knew how the stories would unfold from the start otherwise for such a complicated series the writers would have been forced to show Sisko waking up in the shower realising it was all a dream! But in terms of just sitting down in front of a random DS9 episode the lay viewer would be screwed, "Jem Hadar who!?!", "Who's on which side of the war now?".
Voyager was the antidote to DS9, you could take nearly any episode and just sit down and watch it with just the barest notion of what Trek is about to understand it. Indeed most episodes could be put in any random order and it wouldn't make a difference. The only long overarching story (besides getting home) was Tom Paris' receding hairline. I liked the fact that you could just sit down and get your shot of Trek espresso but it contributed very little to the understanding of the Trek verse. The Delta quadrant is still as much of a mystery as before.
TOS is just too old for me to take seriously. I just cannot look past the shoddy production values to appreciate the storyline. I'm afraid that I'm just too spoilt as a viewer to accept it. It seems more like a draft for the later series. As someone on this list has already said before, Enterprise was an attempt to rewrite trek history. I just didn't watch enough of it to comment seriously on it but I don't think they really needed to bother with this series.