Since we all like to make lists:
1. TNG: best written series, likeable cast all around.
2. TOS: at the time it was brilliant, and some episodes still shine on to this day.
3. DS9(action): pew-pew!
4. ENT: had potential, but got cut short right when it was building up; best pilot ep of all; definetely grows on you.
5. VOY: inconsistent, cheesy, premise doesn't really work, most characters are either bland (Kim, Janeway) or annoying (Neelix, Kes, Chakotay, B'elana, Paris)... you know you've got a problem when the most interesting character is the emotionless Vulcan.
Very few stand-out episodes.
6. DS9(religious propaganda): as terrible as "7th heaven" at times, and that's saying a lot.