both of them, but I do slightly prefer DS9 and believe it to be the best Trek.
That being said, DS9 had the benefit of taking place in a universe that was built on two previous series. You can't forget that without TNG, there would be no DS9. As a series, it was able to hit the ground running without (too much of) an awkward phase, because TNG laid a path for it.
(TNG also laid a path for Voyager, but instead of exploring off the path, Voyager chose to wear it into a rut.)