There’s a lot of good general rules but I do think at times it can really depend on the game, especially between different genres or scenarios, how important it is at eliciting certain responses from the player, whether the game is meant to be super challenging or a bit of a breeze.
Everyone knows Fromsoft these days, and while they were generally pretty good at doing many of them, I’d argue Elden Ring has a ton of design flaws as a result of trying to one up expectations for challenge. Essentially put themselves in a corner with the most annoying fanbase ever, and it got in the way of designing something to just be good in the first place.
Platinum is one on a similar level. Not always perfect for sure, but when they get something right it’s usually a pretty good time and just the right amount of engaging without feeling impossible or rude. I feel the first Nier had some bosses of a close enough caliber that Platinum was earnest in developing its sequel. That being said, I feel both games were designed to be on the easier side while still giving that feeling of being cool.
Generally I feel both Undertale and Deltarune have some really good stuff. Most of those games are more about the moment to moment interactions with goofy characters and experiences, so the boss fights are usually more about expressing that through little gimmicks and such. And if you want harder bosses, you certainly can have those as well.
I feel a lot of Kirby games get overlooked for this aspect, but maybe I’m wrong in that I just don’t see it anywhere. I usually just take them on without any power ups, knowing there’s an intended way to do things, but the credits to Kirby’s Adventure had shown just how creative people can be if they’re skilled enough. I certainly had a tough but fun time with Forgotten Land’s arenas in a similar fashion.
Metroid Dread…some of those really did a number on me, making me come back the next day on the last one. But at the very least, they were usually very satisfying and very thrilling to fight. Far and above the other 2D games in the series. Kinda hard to compare with Prime’s slower pace.