TNG is my foremost love among Star Trek series. It has an unquenchable optimism about it. The society it depicts, or the very small part of that society that we see on the Enterprise-D, is such a goal: the productive capacity of a society leaving no one with basic material needs unmet, people working to grow in one way or another, colleagues who trust and respect each other, a room where you can ask a computer to show you naked ladies in living color...the things humanity could do if we worked together. I have trouble imagining what a socialist utopia might actually look like, but TNG gives us an inkling.
I came to DS9 later in life. While I recall watching TNG even before I really understood what was happening in the episodes, I shied away from DS9 for a long while. Part of that had to do with a rejection of nerdy stuff while I was trying to "find myself" as an older child, and part of that was it just looked weird to a young TNG fan--new aliens, a space station rather than a star ship, etc. I eventually ended up watching my way through the series during graduate school and enjoyed it a lot. There's a lot of interesting stuff happening where the vaguely socialist society bumps up against societies that are decidedly different.
I have less experience with Voyager than TNG or DS9. Part of that has to do with some associations with people I don't like very much, but the parts of our lives interact in weird ways. The episodes I've seen range from very enjoyable to very bad, which is to say: like the other shows. The unfortunately vivid memory I have is the episode in which two characters "evolve" into lizards and mate, but Star Trek is always up to some sort of hijinks. TNG did that first anyway.