Honestly for me TNG, VOY, DS9 all had their "bad seasons" and they all had "great seasons" It is too hard to pick a clear winner Overall all three were enjoyable .
Throw in Enterprise into the mix and you had 4 great series overall. What killed Enterprise was the initial two seasons. I used to hate the show, then I decided to finally watch it, it was much better from season 3 onwards.
The two new series haven't started great but I will give them the benefit of a doubt to correct their ways so to speak kind of like Enterprise did.