I voted for DS9, without hesitation. Why?
(Warning, this is primarily justifying DS9 over TNG. TOS and Voyager just blew, IMO, and Enterprise hasn't been on long enough)
1) The supporting cast on DS9 was better than any of the others. Not that the others had much of a supporting cast, but the very nature of DS9 being on a space station allows for more of one.
2) The arc. I would have liked TNG a lot more if there was a point to it. Conversely, it's exactly why I hated Voyager so much - they had a very interesting premise and destroyed it. No, DS9's arc wasn't that original (or original at all, if you believe in the DS9/B5 ripoff), but at least teh show had a direction.
3) Avery Brooks is the man. He's what I imagine when I think of a captain.
4) I found the characters to be a little more interesting than TNGs, and a little more realistic I think. I just don't see a man turning down captain-ship three times because he likes where he is. Everyone was a little too goody-two-shoes for me on TNG.
-1) On the other hand, I hated the Ferengi and Holodeck episodes. Especially the Ferengi ones, which made them more slapstick than the greedy, coniving little bastards they usually are.