Really hard to see Buffy/Angel belonging on this list. Too schlocky and too much filler (even if it's often well-done as far as that goes).
Strong disagreement on that. What you call filler, I call character-building.
This is actually one of my main beefs with the modern Trek shows (Discovery and Picard – I'll use Trek as an example because there isn't a modern version of Buffy): the crew are basically just extras. Since the seasons are so short and focused on the Big Story, there's no room to breathe and get to know anyone who isn't Burnham, Saru, Stamets, or the captain of the Season. Or maybe Tilly, a little. Discovery in particular has tried multiple times to make things "interesting" by doing something with another member of the bridge crew – Airam's sacrifice when corrupted by Control, Detmer needing to get her mojo back after crashing the Discovery when exiting the wormhole, and Owosekun being a badass retaking Discovery from the Emerald Chain. To me, all of these fell flat because
I do not know these people. Compare this to Deep Space 9, where we get to know not only Sisko, Kira, and Dax, but also Bashir, Odo, Quark, Rom, Nog, Jake (OK, I could mostly do without Jake), Worf, Garak, O'Brien, Dukat, Damar, Winn. These are all interesting characters that we get to know and love/hate over the course of the series, precisely
because of those "filler" episodes.
Modern short seasons work well when you only have one or at most a handful of important characters and the rest are literally there as supporting cast for the main characters. But they make for poor ensemble shows.
But the showrunners said no they could do it in just a handful of episodes....and they were so rushed it felt like absolute hot garbage. Now I can respect that the guys were tired, or maybe wanted to do something else, or had a new opportunity, etc....these things happen. But you turn the reigns over to someone else, wish them good luck, and let the project continue...you don't just kill it so you can be over and done with it.
My understanding was that Disney wanted D&D to do Star Wars, and they were all over that, and so they rushed to finish Game of Thrones. Of course, Game of Thrones season 8 ended in such a mess that Disney then told them "On second thought, no."