I'd have to say TNG overall, although I can't really stand Star Trek anymore, especially the new one with Scott Bakula. Personally, I think the best thing about Star Trek is the early movies, with the exception of the absolutely dire Star Trek V, of course.
DS9 only started to become good when they introduced the Dominon War, which was obviously a blatant attempt to emulate the wave of success that came from Babylon 5's Shadow War. (Indeed, many would argue that the whole concept of DS9 was essentially lifted from Bab5, but I don't think there's a lot of basis behind that claim.)
In fairness, though, for all the adulation Babylon 5 recieves, it was also pretty crap until they introduced the war. Just goes to show how incapable most sci-fi writers are these days at constructing interesting stories where the characters aren't blasting eachother with phasers for 40 minutes.