TNG had a good run, there's no question about that, but I would not say that its the best Trek series. Granted, it had its fair share of good episodes and while the actors were good, especially Picard Stewart and Brent Spiner's acting (with Marina Sirtis being a liability), the chemistry between them was never like TOS cast. They just were not as chrismatic as TOS cast and this in part has to do with the fact that TNG characters are not very interesting to begin with. This is not TNG cast fault, its Roddenberry's because he created these characters who came across as bland, righteous, preachy people who were true blue heroes who could do no wrong and always told the truth. They were essentially black and white characters with no shades of grey. Essentially, they were cardboard caricatures, not real human beings with flaws, warts, vices and all that good stuff. DS9 got that right and to an extent, even TOS, they were more real to me and that's why I can relate more to these people as opposed to TNG. But what's worse, TNG had no interpersonal conflict, thus, making the show very boring at times. Conflict is the very essence of good drama and if Roddenberry wanted to write for a show that featured no interpersonal conflict, then he would have been better off writing for Sesame Street!
In a lot of ways, DS9 is a more worthy successor to TOS than TNG because both TOS and DS9 proved that despite the characters flaws, differences and internal conflicts, when push came to shove, they all worked together as a team and pulled through for each other. An example of this was the love-hate relationship in TOS involving Spock and McCoy and DS9 involving Quark and Odo. They had their differences but they always had a certain respect for each other and save each other lives many times. Michael Piller who wrote for TNG and DS9 said it best, "people who come from different places—honorable, noble people—will naturally have conflicts".
Furthermore, in one interview prior to TNG television premiere, Roddenberry stated that the relationship between Picard and Riker would be a more closer one than Kirk and Spock. Well, in retrospect, that never happened. While the relationship between Picard and Riker was that of respect and admiration, theirs was always more formal than Kirk and Spock. Kirk and Spock hugged each other, went on shoreleave together and even sang campfire songs together. They were more than just friends, colleagues and shipmates, they were family. That was never the case with Picard and Riker and I wished they had a deeper friendship than the final product. Hence, TNG cast did okay on tv, but barely because their chemistry felt forced and they can't carry a film to save their lives, their track record is proof of this. What's ironic about this is that I used to watch TOS during its reruns, but I was too young to understand it at the time, however, when I was a teenager, it was TNG that made me into a full-fledge fan. Now, seeing TNG through the eyes of an adult, TNG is not my favorite trek show anymore, its DS9, but I will always treasure TNG for the good memories of my youth, anyway.