In the world of human-readable data formats (ie not programming languages), the best one I ever used was Jane Street's sexplib[1] s-expression format.

It was concise and expressive. There was a direct way to describe variants (types with multiple constructors), which is always awkward in JSON, but the format was still surprisingly low-noise for reading and editing by hand. I remember you could even use it as a lightweight markup format:

    (here is some text with (em formatting) information)

(The format leaves the interpretation of things like (em ...) up to you; you could use it as a slightly more verbose Markdown, but you could also use it to structure readable text with other sorts of metadata instead.)

And, unlike certain other formats I won't name, it has comments!

It also helps that Emacs with Paredit makes editing s-expressions flow. The tool doesn't need to know anything about the sexplib format specifically; just relying on basic s-expression structure gives us fluid but simple structural editing.

I am constantly sad that nobody else uses this sort of format, and I have to deal with a mixture of JSON, YAML, TOML and other ad-hoc formats instead.

[1]: https://github.com/janestreet/sexplib