About at the end of Season 3 of Star Trek: Enterprise (second time watching it through) and I have come to realize that it is probably the best Star Trek series as it actually has a plot arc rather than just single episodes. Only really Voyager did the same/similar thing but their "arc" was mainly to get home.
It's a shame that Enterprise was cut short at 4 seasons (main reason I read/heard back then was low ratings due to baseball shifting the time slot a few times).
The Next Generation was my favorite forever and I only watched the first season of Enterprise when it was airing (was on UPN, not even sure what channel that is/was, and I believe it was before DVR). I also never watched Voyager until sometime last year.
Now I would rate them:
Deep Space 9
I am sure I am missing a series in there somewhere but those are the ones I have watched from beginning to end.