I it I was initially unimpressed with Immortal Days because this certainly wasn't the first time there's been a manga about mortals living in an immortal world, but this series exceeded my expectations and then some, to the point where I'm waiting in anticipation for each new chapter.
The characters are all very well-developed and thought out, and even minor characters seem to have their own stories going on in the background. In general, the author seems extremely good at worldbuilding, and there will be little tidbits here and there that really add to the world, without making it into an infodump.
This is also one of the few series I've read, in any medium and language, that's just so... effortlessly diverse? Like there's people of all skin tones and ethnicities in all kinds of roles, and also queer people are just... here, without comment. They even have gender-neutral school uniforms!! It also does eventually address things like discrimination, etc. (Like, actual discrimination, not allegorical fantastical discrimination.) And while I wouldn't say it does it perfectly, the author is very clear with their intentions and framing, which is a breath of fresh air -- no fence-sitting here!
All in all, I'd have to say that Immortal Days is one of the most impressive series I've read in of worldbuilding, and while the overarching plot can get a bit scattered, the characters more than make up for it with strong characterizations of their own.