There are many lists with recommended/must-watch episodes. But they usually just include episodes that are considered good or great.
Is there also a list with episodes that are important but not necessarily good? By important I mean episodes that introduce new concepts or new characters (or kill off important characters), or that have lasting repercussions in the overall story of the series or Star Trek as a whole, or are necessary to watch first for a pay-off way later.
For example, TNG’s “Skin of Evil” shows the death of a main character but it’s hardly a good episode. The same goes for DS9’s “Life Support”. I wouldn’t exactly call ENT’s “Storm Front” good either but it’s important to cap off the temporal cold war. TNG’s “Ménage à Troi” shows Wesley being promoted to Ensign but the rest of the episode is rather mid (except for the Picard meme) so that it wouldn’t ever appear on any must-watch list. etc.
I see your point. What I tried to assess is: which episodes enhance the viewing experience by rewarding the viewer for sticking around. For example, watching “Datalore” before “Brothers” is definitely not required. But it gives you some good backstory on Lore. Similarly, watching the various Maquis episodes on TNG and DS9 is not necessary to understand the Maquis characters on VOY but it’s a nice slow buildup that contextualizes a few things. Or TNG’s “The Price” and VOY’s “False Profits” – neither of them are really good episodes but it’s a “oh hey, they picked up that storyline from years ago” moment that I feel is very rewarding.
And of course my all-time favorite joke when Worf hears that Keiko is having another baby and he reacts with “Now!?!?”. It only really works when you’ve seen “Disaster” before.
I do love a sequel years later out of nowhere! Ship in a Bottle is also a good one for that.