JSON doesn't have comments so it's a bad choice for human-editable config. YAML doesn't have an end marker so you can never be sure if you've got the entire file. XML is a huge pain to edit by hand if the schema is complicated, and overly verbose if it isn't. None of them are even close to being safe (for example https://arp242.net/weblog/yaml_probably_not_so_great_after_a...). All of those choices fail your "elegance" test.
TOML is my preferred config file language option where I have a choice - https://github.com/toml-lang/toml - but I suspect that suffers a lot of the same problems.
TOML is my preferred config file language option where I have a choice - https://github.com/toml-lang/toml - but I suspect that suffers a lot of the same problems.