The declarative form of clap is not quite as well documented as the programmatic approach (but it's not too bad to figure out usually).

One of the things I love about clap is that you can configure it to automatically spit out --help info, and you can even get it to generate shell autocompletions for you!

I think there are some other libraries that are challenging it now (fewer dependencies or something?) but clap sets the standard to beat.