The worst config files I've ever encountered are dynamic YAML templates for Kubernetes. Mind boggling to figure out with crazy indentation rules and for loops. Kill me now.
Seriously, the more time I spend with other configuration formats the more I start appreciating JSON. It is a simple array and object format. Nothing can go wrong with it. No indentation rules. Easy to encode and decode. Easy to turn into actual arrays and objects in your programming language. Lack of types is not great, but use a type checker in your parser that throws exceptions and you are fine.
Seriously, the more time I spend with other configuration formats the more I start appreciating JSON. It is a simple array and object format. Nothing can go wrong with it. No indentation rules. Easy to encode and decode. Easy to turn into actual arrays and objects in your programming language. Lack of types is not great, but use a type checker in your parser that throws exceptions and you are fine.
JSON is simple and that is why it is successful.