I prefer original series and TNG to later series because of what they were, though I can appreciate that different people like the different shows for their own reasons.
To me Star Trek was always the crew being presented witha unique sci fi situation every week, and a story showing the interesting implicatins of dealing it. Anything from a planet where women were alphas, cowboy planet, drunk virus, people becoming insubstantial, evil mirror world, whatever. In later series, starting with DS9, they went away from this formula, concentrating more of an overreaching arc that dealt with more issues like space diplomacy and interactions of various cultures with each other. It's interesting, it's just ... not classic Trek to me. Some of the most popular TNG episodes do this, like almost all the Borg stuff for example, so obviously it was a good formula. Simlarly the movies ar emore like this, since you can't really have a movie that's just about some sci fi thing and they just deal with it. It's a format that works better for TV.