[martypitt]

HOCON is a worthy contender in this space - I wish it got more airtime. (We use it extensively).

JSON superset, optional quotes for keys, sensible string handling, comments, automatic env variable handling, variable references.

It's not perfect (all sufficiently powerful configuration language has quirks), but I love it.

[asimpletune]

We used hocon at a place that I once worked at and I more or less liked it. It did get a lot of abuse though. I think apple released a configuration language that seemed pretty good for the same things that we used hocon. I think it was pkl or something?