TNG takes a bit to settle in, though there are some early classics in the first few seasons. S4 onwards it gets good.
TOS on the other hand goes downhill after the first season (and certainly the third).
DS9, from when the Jem'Hadar are introduced gets good. A bit meh about the last two, certainly the last. Should have just gone all out with the one story the whole series and a much bigger ending, though I get the feeling they were hoping for a movie spin off.
VOY is consistently meh all along and some stinkers. Though it's averagely watchable. The Fair Haven stuff... good god, no! Worse than Vic Fontaine in repeated annoyance and pointlessness. Has a candidate for all time worst ever Trek episode as well, at least to me, 11:59. Zero to do with Star Trek and is at best a boring daytime telly snoozefest.
ENT - Only pre-JJ series I've not been interested in buying on DVD/Blu Ray and not re-watched. It got some good ones later on and had potential, but I just wish it had never tried to fit in as a prequel. It restricted it and didn't fit anyway. Maybe it will grow on me with a re-watch.