Star Trek TNG finally found its mojo after a few seasons, but I prefer TOS because its best writing (Amok Time, Mirror Mirror, Doomsday Machine, Trouble with Tribbles) was not equalled in any later series' episodes that I saw, and because the "trinity" casting coup (Shatner/Nimoy/Kelley) provided a perfect chemistry. Only two regular actors in TNG (Stewart and Spiner) were of equal or better caliber but they didn't actually interact with each other that much. Also, TOS has not only the best musical score of any Star Trek show, but the best musical score of any TV show, period, that I have seen. On the down side, TOS's lows (Spock's Brain, Catspaw, Lights of Zetar, Omega Glory) were probably lower than any other Trek series -- although TNG had some real stinkers. (Remember the little "videogame" that takes over everyone's brain on the Enterprise, and only Wesley can save the day?)
There were a few episodes of Voyager I really liked (the "Tuvix" episode is one of my favorites), but a lot of the supporting characters (Kim and Paris for instance) were dull, Neelix was annoying, and Chakotay was underused. I didn't see enough DS9 or Enterprise to make an informed judgment on them, though DS9 seems to have been the more promising show of those two.