DS9 for me, and for a very simple reason.
It's the best representation of what the franchise is all about. A station manned by two different organizations, crewed by people from dozens of worlds, many of whom are outcasts or exiles from their own culture (Odo, Worf, Garak, Quark for a while)... all working together and learning together. I can even point to a scene that says this loud and clear.
At the end of "BODY PARTS", Quark is talking with Rom in his empty bar: all his possessions were taken by the FCA. Suddenly, Bashir comes in with alcohol to help him get back on his feet. Then Dax with glasses. Right after that, Sisko and Odo come in with everyone, bringing furniture and assorted stuff to get him back to business. Morn, his most loyal of patrons, comes to sit at his usual spot. Quark is stunned at realizing that he had assets all along... Sisko, Dax, Bashir, Morn, etc. And just when he was about to say something, he just stops and smiles. He clearly had an epiphany: that not only did he belong there, but he was a part of the community and was loved.
That might be the one single scene that exemplifies probably the most core value of the franchise: we are all in this together, we are all welcome, and we all belong.
And DS9 was full of scenes like this. The premise of the show itself is a clear statement that it's following the ideals of STAR TREK.
I've got other reasons, but as someone who grew up being very much an outcast to everyone, this speaks to me the most. Which is why DEEP SPACE NINE, more than even TNG (which is basically why I became a scifi fan), is my show.