Though all the Star Trek shows have their strengths, appeals, and loyal fans, if I were to rank the shows it would be like this.
TOS=TNG>DS9>ENT>VOY
TOS and TNG are the core, the very foundation of the Star Trek universe and franchise. At the end of the day, Star Trek is about “boldly going where no one has gone before” in an adventurous, optimistic, and entertaining way. TOS and TNG execute this concept the best, so these two shows are tied for first. (Also these two shows were the most commercially successful).
If one designates TOS and TNG as the core of the franchise, then DS9, VOY, and ENT would be the spinoffs of TOS and TNG. (Though DS9, VOY, and ENT can all be enjoyed as stand-alone shows as well.)
So when ranking the subsequent three series, I asked the question, “To what extent does the show broaden, deepen, and enrich the Star Trek universe set up by TOS and TNG?”, and I came up with the following ranking; DS9 being the first, then ENT and then VOY.
Some may wonder why ENT, the show that got cancelled with just 4 seasons, is ranked above VOY which got its full 7 seasons, so here is my reason. VOY, though it has its strengths and appeals, is ranked last as a spinoff series of TOS and TNG because the show played things way too safe. It never took any risks and ended up looking too much like TNG Lite. Though that may be fine if you watch VOY as a stand-alone show, being TNG Lite is not a virtue if you judge the show as a spinoff series. At least ENT tried to be something different from TOS or TNG. VOY never even bothered to try.
Just so there is no misunderstanding here, I’m not saying that Voyager is a bad show. As I said before, VOY has its strengths as a stand-alone show.