I love Deep Space Nine a whole bunch... but if we're going to be honest it wasn't as successful as TNG because for most people it wasn't as good as TNG.
And "The Wire" and "Breaking Bad" haven't been as successful as, say "White Collar" or the new "Dallas" because most average American consumers would likely support the latter group over the former. But does ratings and fan approval represent quality? No and it never has. Some of the very best and most creative TV shows end up lasting a season or less while some of the most ordinary television go on forever (Grey's Anatomy, Criminal Minds, CSI). I suppose it is all subjective anyway but nonetheless I can make a claim that using ratings success as the means to determine the quality of a show is absurd.
Now I will concede is that there is a bit of revisionism going on about the serialization of DS9. Babylon 5 was a true example of a serialized show; DS9 on the other hand never came close to that strict a format. How could it. It was a Trek show and therefore had to follow some previous guidelines as well as follow the orders of the studio which financed it. Nonetheless DS9 did have serial elements that no other Trek show came close to displaying. It relied heavily on continuity and long story arcs.
Personally to me it is the best Trek and I say that as someone who loved TNG like crazy. But I felt DS9 was simply something special back then and to be frank it holds up a lot better now than TNG. Just my opinion but I'm not alone. Since DS9's ending there have been countless people in articles that could have been found online at genre or TV websites and in genre magazine. The only mainstream affirmation that I recall came from TV Guide which wrote, when DS9 ended in 1999, that it was the best of the modern Trek shows. Plenty of people feel that way.