I wouldn't say TNG is the very best, personally I feel DS9 was better overall. TNG has the best captain, but sometimes it was marred by silly parts of the plot like Wesley saving the day or Dr. Crusher having an emotional crisis.
I think TNG and TOS are typically the most highly regarded.
I feel TNG is the best by a long shot! You're right that it does have a few "off" episodes, but it also has more truly memorable ones. I feel that DS9, while good, has less truly memorable or standout episodes.
DS9 is my favorite but only by a relatively small margin. Most of the same make up artists, costume designers, writers, and directors were involved in TNG, DS9 and Voyager. Most of my problems with TNG are from the crew going through a learning curve and most of their newbie mistakes disappeared by the time they got to DS9.
Sir Patrick Stewart is the best actor that ever graced a Star Trek set and his very presence elevates the acting of the others around him. In a way he set a precedent for how future Star Trek officers would act for many years much how Robert Downey Junior set standards for the Marvel Cinematic Universe through
Endgame.
The reason I like DS9 is that in my opinion, DS9 had no dead weight characters (MAYBE Ezri Dax, but she was mediocre, not awful). Tasha Yar and whoever the snide doctor who mocked Data was were put on a bus because no one liked them. Wesley eventually got put on a bus after annoying the fanboy. Each episode of TNG the writers flipped a coin to see whether Troi would be competent or incompetent.
Worf got humiliated so much TV tropes named
a trope after him. Both Worf and O'Brien were a lot better written in DS9. The cast of DS9 was thematically linked. In a way they were all lost and adrift and they found a surrogate family in each other. I also really liked the non-Federation perspectives provided Garak and Quark. Voyager continued this tradition with the introduction of Neelix. Not quite as good as Garak or Quark, but close enough.
A lot of it's personal taste. DS9 had more politics and fewer weird alien creatures than TNG because most of the series was set in a stationary place, but I like sci-fi politics. I liked how yesterday's enemy kept becoming tomorrow's friend and visa versa.
Other people prefer episodic space adventures in weird places and that's fine too.
This is the way...to candy.
For those uncultured swine,
this is why it's funny.