DS9 is my favorite Trek and I think the best of all the spinoffs because it's the most human. I get the argument that some fans state they dislike how DS9 went against Gene Roddenberry's utopian views of Trek and yes it did, but personally I like the writer's view that yes Earth is a paradise but the rest of the universe is not.
Why DS9 is so successful for me is that it, as spinoffs should, dared to be something different than what had come before it. It took some big risks some of which worked and some that didn't. This to me why Voyager failed in my book instead of trying to be its own thing, Voyager decided to become TNG Part 2 (and of lower quality than TNG was). And ENT don't get me started.
I also think out of the 90s era of Trek, DS9 has aged the best. Partly because the topics the show covered (war, religion and terrorism) are very relevant in our post 9/11 world. Voyager, which has great FX, to me felt dated and old when it the episodes were new.
That's my two cents.