I'll stick with JSON for one simple reason: you can have anything as a key (as long as it's escaped). The fact that this can't means you either a) can't use it if you have more-complex keys for some reason, or b) you have to use some non-standardized packing format to encode your illegal keys.
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edit: thanks repliers, I missed part of the spec. Ignore!
2.2. Objects
An object structure is represented as a pair of curly brackets
surrounding zero or more name/value pairs (or members). A name is a
string. A single colon comes after each name, separating the name
from the value. A single comma separates a value from a following
name. The names within an object SHOULD be unique.
> Note that keys and values can be elements of any type.
The sample code also uses a vector as a key.