I liked DS9 the best by far. Only series with a real story ark. Only series with decisions and actions that mattered for more than one episode. Besides it had the only ST villain who left an impression with me, Gul Dukat. Gharak and Sisko where quite likable too, and non of the other characters stood out as particularly annoying (which is a huge deal!).
Voyager had the best premise but completely blundered it. They turned what should have been a rough and uncertain journey home, bringing people and ship to their limits, into a happy-go-lucky trip. I would so have loved to see the morals and principles born out of the ST post-scarcity society collide with the harsh realities of survival in hostile territory, but no. Only episodes that came close to that were Equinox and Year of Hell. Besides, except for the Doctor there were no really interesting characters in this show.
TNG has some strong episodes and some great characters. But the overall principle is taken over from TOS, resulting in little to no premise or continuousness, its basically a constant deus ex machina mixed with "character episodes". And some of those characters...Picard and Data are cool, but what the hell is up with...everyone else? Worf is an
incompetent warmonger, Geordi an equally incompetent
punching bag, and Deanna serves no apparent purpose whatsoever (Enemy threatening to destroy Enterprise: "I can feel hostility captain!").