Between XML, JSON, YAML and TOML, I would rather my configuration files served in XML, with schema validation and graphical IDE tooling, thank you very much. :)
I personally prefer JSON to XML. I've seen XML abused in some pretty terrible ways. For example, I worked at an education start-up several years ago and the education company Pearson delivered bubble test questions to us in XML where half of the questions' text were in tag attributes and the other half was in the body of the tag. JSON (and YAML/TOML) make it a lot harder to do stupid things like that. They're also easier to parse, in my opinion.
