TOS changed the way people thought of scifi (still havnt watched the whole thing through), but TNG changed the way people thought. Even early on Gene was using aliens to talk about narratives that other show writers weren't willing to touch. Which brings one of my favorite episodes to mind, the androgynous race and their A2F woman, and Rikers mad crush on her. That, and ALL the Barclay episodes
DS9 was a tad slow, not being able to leave their station for the first 3 seasons, but that did give room to great character development and world building like we hadnt seen before in a star trek.
OMG, Voyager!, Janeway is SUCH a strong female role model. Plus, the idea of being cast out to nowhere familiar, struggling to survive, watching resourses. Gives the Star Trek model so much more of a realistic feel. Akin to Frefly, which I suppose is for another thread.
Havent watched the new ones yet.
It's been a long road...