Config files should be readable and editable by both humans and machines. What defines this? A format with a clear specification (for the machines to parse) and easy on the eyes/fingers (for humans to read and modify). There are a few popular options:

• XML: Basically HTML - tricky to read and tricky to edit by hand. Diffs will be messy.
• .INI: Several competing specifications, so really not even worth considering.
• YAML: An improvement over XML in readability, but the whitespace-as-syntax might trip you up. GitHub Actions uses this for defining workflows, and tools like Dependabot and pre-commit read configurations from YAML files.
• JSON: The de facto format for passing around data in webdev, with a dictionary-like syntax. Hope you like commas and braces.
• TOML: A happy medium between human and machine readability. Used for specifying Rust project dependencies in Cargo.toml, and for Python project configs in pyproject.toml.

I reach for TOML if I have the choice.