DS9 is my favourite. The whole Dominion War story arc really put that series over the top and some of the individual episodes that composed it were incredible.
TNG -- the more I watch it, the more I dislike it but there are some stand out episodes.
Enterprise -- I liked it, especially when the show was cancelled and the writers used the last few episodes to go nuts with some interesting ideas. Pity the first two seasons weren't like that.
TOS -- I'm beginning to hate it.
VOY -- I hate it. Except Seven of Nine. Wow.
All of this is moot...the best sci-fi series is the new Battlestar Galactica