There are a bunch of data formats which can store structured data. The most popular seem to be JSON and YAML. A relatively new one is TOML, which is gaining traction in the Python ecosystem. For most of my projects I just use YAML files, but I have tried out the TOML language as well. //

Another downside of JSON is that it doesn't support comments. TOML and YAML do that. However, it is hard to re-write such a file with comments and change values while keeping most of the comments intact. But this just means that commented files are only read by programs, and if programs write the files, one should not comment them.

YAML
The YAML format is my definite favorite. I find it easy to read and write. One negative about YAML is its potential complexity, as one can have references to other part of the file, or even serialize custom types.