No doubt Doctor Who
Being the longest show, this has varying levels sometimes its greatness, sometimes its repetitive and sometimes its boring.
For example, the first time they showed those weeping angels, that particular episode was very intense. Later they became more like meh.
I have also watched most of the old, 60ies to 80ies, and there also sometimes its great sometimes meh. Some characters you like and enjoy, then they get boring or they just leave and the next (companion/doctor) is annoying.
Perhaps you could say the same of the Star Trek franchise, but i guess it can't be helped with episodic shows. More recently (started with Babylon 5) sci-fi shows have become less episodic and more main (long) plot based, this is good except for the people catching them in the middle, which is less of an issue now that we are entering the streaming age and TV is becoming a thing of the past.
From the aforementioned, I vouch for The Expanse, but have yet to see the others. Sometimes i wait for the whole thing to end before starting to watch...