TNG.
My reasons:
1. The entire thing hasn't aged well. Seasons 1 & 2 are practically unwatchable now, and the rest, while better in this regard, still feels very '80s.
2. I think that out of all the Trek shows, it is least like the the Original. Maybe the format is the same, with the whole 'alien of the week' thing. And the ship is named "Enterprise". But there the similarity ends. Because TOS presents us with flawed heroes as the centerpiece of the series...and it is these flawed heroes who manage to make a difference - learning for themselves and instructing others at the same time - not only through their words, but through their own mistakes. In TNG the heroes are already perfect, and so 'regular army' that they are boring and one-dimensional. And they do not approach problems like the TOS crew did...because there is no trial & error on TNG. They already know EVERYTHING. And will be happy to tell anyone who asks as much.
Which leads me to....
3. I really, really, REALLY cannot stand that smug superiority that characterizes the TNG crew. The view that they are perfect, can do no wrong, and that they are there to 'instruct' other races in the 'correct' view of the universe is exceedingly annoying. And incidentally, has become even MORE annoying during my own country's current Presidential administration.
4. Some of the TNG actors are really annoying. Marina I-can't-get-a-job-outside-of-Trek Sirtus is especially so, running around making nasty comments about other Trek shows when she was clearly the weakest link, by a LONG, LONG margin, on her own. Never has an actress who was such an incredible waste of space been so very proud over the fact.

Because if it hadn't been for TNG, Marina Sirtus would probably still be saying "You want fries with that?" on a regular basis.
Her and Frakes hogging the ENT finale didn't help my opinion of either of them, by the way.

ENT may have not been the best Trek ever or anything, but that cast AT LEAST deserved the dignity of their very own send-off. But Frakes and Sirtus were such money-grubbers that they didn't for a moment put themselves in those actors places. Instead, they were quite happy to rush right in and hog the entire finale for themselves.