I think you can make a convincing case for TOS, TNG and DS9.
DS9 obviously didn't make the impact on popular culture that TOS did, nor did it ever achieve the recognition and popularity that TNG attained (and retains, at least to a degree).
It also benefited immensely from the groundwork laid by those two shows.
A lot of what makes DS9 interesting is the way it revisits some of the same stories and ideas from previous Trek shows, but from a fresh perspective. In some cases, long-time Trek writers were consciously doing this, while questioning some of the utopian ideas in Trek that had become almost a sort of dogma on TNG. For that reason, DS9 has aged much better than TNG, and I think it will continue to do so.
The main case for DS9 rests on the overall quality of the writing and acting, the depth of characterization, and the willingness of the producers to experiment and try new things. It's an uneven show, but at its best it can be really great.