We tried to use YAML at first, but problems with data types (can't correctly remember what, but there was no way to force something to be something) made us to rewrite our test fixtures to JSON. The only problem with JSON for us is that it doesn't support any comments, but JSON5 seems to fix it.

There's a interesting format for configuration called TOML[1], you should check it out!

[1] https://github.com/mojombo/toml