You asked me my favorite Trek series, so I'm curious what's yours 
Haha, very fair!
So unlike you I did watch all the series so far. I don't have any nostalgia towards any series as I only started watching everything a few years ago, starting with TNG, then TOS and then everything else, in order. Just missing a few of the movies since they are somehow not on Netflix? Anyway.
I think my favorite has to be DS9. It really grew on me over time. The characters are the most genuine I have seen on a Star Trek show and for some reason, even though most of the series takes place in the same location, the plot was extremely interesting. It was also the only show where an episode (S4E2) made me completely bawl my eyes out. It was a random episode, not part of the main plot, and completely caught me off guard haha.
However, that being said, I did enjoy the entire thing immensely. TOS is rather sexist in today's point of view, but it still holds up pretty well. Their love for exploration definitely shines through, and I can see why it inspired a whole generation of fans. TNG is good, great cast and stories. Voyager had a "meh' ending but the story itself is amazing, and Janeway is an awesome captain that I look up to. I was pleasantly surprised by Enterprise (except its theme song lol). Really upset that got cancelled right as it started getting good. Probably some of the most relatable characters on that show. And as for the newer shows (Discovery and Picard), they are aesthetically pleasing, but haven't really "caught" me yet. Hope they can turn that around.