Everything was great until the Xindi saga... no Star Trek series should have huge, season-long story arcs. That's Babylon 5's turf.
Personally, I always found TOS and TNG a bit cheap and cheesy. Too many episodes revolved around an away tem beaming down to a planet, talking tot he natives, beaming back up to the ship, talking some more, beaming back down to the planet, doing more talking... and so on and so on, with nothing actually happening. I found DS9 and Voyager vastly more interesting, especially when the Borg were involved. I'm a major Borg fan. And Seven was a much more interesting character than Kes. "Ensign Kim, take off your clothes!" heh heh... great stuff.