As a general rule, I'd say seasons 2-5 are the sweet spot (not counting kid shows, though sometimes it's true even then). That could be one to all of the seasons in that zone. (Also generally speaking, I think this is true of many book series as well.)
But there are exceptions. I say ST: NG was best in season 6, but 4-6 are its sweet spot. (And the last season, despite slipping badly, was still better than the first 3.) I think ST: NG could've had a better s7, but they made some quick changes on the fly to be compatible with the movie. Or something along those lines.
While it's been a long time since I saw DS9, I think the last half was good (more often than not) in the last half to the end. In contrast, I'm only generally fond of the first 2 seasons of ST: TOS (can't remember how many seasons it had--I'm skipping other shows that were exceptions because they were under 5 seasons, and I've gotten tired of looking up info on shows because of the glut of remakes and imitations that I have to sort through to find what I'm looking for, I'm starting to wish there was a law against calling a series or movie the same title as the one it claims to be a reboot of).
Clone Wars got better with each one. I think the final would've been the best had Disney not stepped in, with their best material being released in book and comic form before the last season was greenlit (years later) and needed new material.
Once Upon A Time was my favorite for 1 season and a half (maybe 2 and a half). Then it just got worse and worse. I'd loved it before that I TRIED to get back into it, but at some point I had to give it up. It had become too unwatchable for me. I think they told the story they needed, and then continued on (careless of wild retcons) to beat more money out of it.
There were a couple that I might've saw differently had I approached them at different points in my life. I greatly enjoy, and sometimes rewatch, the first 2 seasons of Xena, but I prefer to avoid the drama and bickering of later seasons. I mainly wanted a fun show there not to take seriously, kind of a kids show in a way. Maybe if I had a lot more spare time I'd have allowed myself to get into the later seasons, but as it was, I've sampled later seasons and generally did not care for it.
Similar, but not the same, for The Walking Dead. The first held me in its icy grip, but after that I just lost interest. But part of that was because it was becoming more of a soap opera with a back drop of zombies, at least it felt that way to me, but I missed too many eps which left me feeling lost on the later eps I got to see. Some arcs sound good, but I just couldn't get into it. Doesn't mean I wouldn't if I watched in chronological order, and maybe my answer would be different if I had. But I have too many other things I want to see (and read) to force myself to do that. I feel no obligation to watch something just because everyone else is.
Again, these are the exceptions to the rule. I think most with more than 5 seasons are best between or through 2-5
(Buffy is close enough to this though not in the specific zone, and Angel is too much of a mixed bag--I liked the idea one had of dividing the seasons into halves to measure them by rather than as a whole--for me to say what I feel about the various seasons of that show.)
