DS9. I love TNG too, but DS9 is the raddest. Less silliness about humanity having "ascended" past all of it's problems in a measly 300 years, actual warfare with nasty consequences, the most interesting characters out of all the series' (with the exception of Spock) and actual plot arcs without everything always resetting at the end of every episode.
DS9, first and foremost, with a distinct nod of the head to TOS (which I watched when it originally aired, though I was a young sprout at the time).
TNG has not aged well, worse than TOS, in my opinion. Voyager never got me. And while I like some of the design sensibilities of Enterprise, I felt it strayed way too far from canon.
DS9, clearly the best for both stories, characters and acting, although Classic Trek will always be special and TNG was very good.
However, I will add that a family friend has loaned us the DVD's of Voyager and we've been watching them as a family. We just finished Season Four. My wife and I had not seen any episodes since they first aired and our three kids (now ages 8 to 13) had never seen any episodes. I never cared for the show that much before but can honestly say it's better the second time around, especially in DVD format because in three or four episode blocks you can quickly dismiss the stinker episodes and go on to a good one. And Kate Mulgrew made an awesome captain!
Spike TV airs Voyager off and on. I never watched it the first time around, and I have to say, its not as bad as I remember. Granted, its not as good as DS9, but its watchable. I've seen a few stinkers, but its no worse than most of the crap on TV right now. its watchable.