It's depressing to read a comment from one ST fan bashing another. Can we all agree on one thing: that the universe of Star Trek changed (perhaps in a modest way) not only the way people viewed sci-fi but also how they envision the future?
Each of the four series is excellent in their respect ways:
(1) ST-TOS for starting it all and for revealing Roddenbury's creative genius. TOS has influenced American culture in many subtle ways ("beam be up, Scotty"). Sure, some of the old episodes were weak ("brain, what is brain?") but compare those to the many excellent, thought-provoking episodes. TOS stands on its own (much like the original "Star Wars") and all other ST series are compared against it.
(2) ST-TNG resurrected the series with fresh characters and technologies (can you forget the always malfunctioning holodeck?). Patrick Stewart was a bold choice to play the captain and the supporting cast was excellent. While the first two seasons were weak, the series was going full steam by the third year. The story lines overlapped episodes (which many of you deride as soap opera-ish, but I loved it) providing a sense of character development. While I love all the ST's, TNG is my favorite.
(3) ST-DS9 overlapped and continued where TNG left off. A darker vision, it also maintained the continuous story line technique. Like its predecessor, it was weak in the beginning but soon came into its own. While I will miss DS9, I feel that seven seasons is enough; any longer and you will weaken the production values.
(4) ST-V departs from the other three series (where the players start from Earth and move outwards), Voyager has an original twist: start far away and revolve the show around the characters getting home (moving inwards). And while there's a certain "Melrose Place in Outer Space" feel to it, Voyager is a fine show (I can't quite get myself to call it "excellent" yet). Starting in June ST-V has a new, critical mission: it will now carry the ST franchise alone.
In summary, I believe that each show has brought its own unique contribution to creating/continuing the ST phenomenon. They are all great creative efforts and each should be admired.
I say down with "the Force" and up with "live long and prosper." Make it so, Number One. Engage.