Which icon sets do you find superior, in particular?

Note that parent meant the UI holistically, and not just icon sets.

AFAIK, this way of thinking about the design of the entire system, and not just in discrete segments is largely unique to macOS. I think it is because Apple has a design language they try to enforce as rigorously as possible, even onto 3rd party developers, which gives the OS & all applications on it a very uniform and consistent feeling.

Windows is the worst. Microsoft has a design language, but they barely encourage it even among their own apps (Settings vs. Control Panel as one example).

I will make a strong claim: I don't think any O.S. developer takes design truly seriously besides Apple. Definitely not Microsoft, partially Google with Android, and certainly not most *nix distros (with the possible rare exception like System76's Pop!_OS).

Most linux desktop environments (notable exception of Gnome3 though it applies for Gnome2) seem to favour old school UX design which, incidentally, I happen to find nicer than “pretty”.

I personally really like the Adwaita icon sets much more than windows and at least on-par with MacOS, though MacOS has the advantage of being rendered only on extremely high DPI screens. So most people only have that experienfe

For whatever it's worth, macOS/OS X has had very high resolution icons well before high DPI displays were commonly available. Even 10.0 which was released in 2001 supported 128x128 icons, which was 4x larger than what other popular desktops were capable of at the time and enabled far greater levels of detail, even if most users wouldn't be seeing that detail most of the time.

Thats actually true with Adwaita also. :D

the default gnome ones, and the whole Adwaita design language. I would prefer it any day over the current mac. Obviously its a personal preference, there's no right or wrong. But I used to like the mac design much more a bunch of years ago. Now, not so much.