I don't know about that- I'm only 4 seasons done so far, but I think DS9 really found it's groove in the 4th season and the writing quality has been pretty good. While the broader strokes of the series aren't that original when compared to B5, it's managed to explore some unique storylines and themes that B5 didn't (mostly since B5 was set in a "real world" future as opposed to a idealistic utopia, so the "easy to be a saint in paradise" angles were much more impactful and easier to explore than in B5's universe) and, unlike Voyager, it's been willing to take chances and make ongoing changes (even if standalones tend to rely on the reset button more often than not). It hasn't been perfect, but as far as my rating scale goes, as Marine pointed out, DS9's "essential" scale are greatly increased as it has progressed. But, moreso than Voyager and Enterprise (which both kinda flailed around without purpose throughout their runs), you can tell they had an idea of what the show was going to be/supposed to be, and stuck to that. In fact, though I'll go into specifics after the season finale when I review the season as a whole, DS9 Season 4 is, so far, the best performing season of the entire franchise, so far as my scale is concerned.