With DS9, we saw the fully destructive effects of the Cardassian occupation on Bajorans, the Federation and even other Cardassians; intergalactic political machinations and manipulations by the Changelings better than the Romulans ever were shown; actually realistic capitalistic villainy in the form of Ferengi (as opposed to their manic depictions in TNG); religious antagonism from some of the Bajorans; actual Federation vs Klingon battles; and on top of that, full scale interplanetary war. I don't think any previous or subsequent series developed the antagonists that well, or really showed the devastating effects of their actions.