In terms of total data transfer? I would assume XML if one doesn't insist on binary. After that JSON? After that maybe BSON, because all the hipster systems run on MongoDB? Otherwise ASN.1 because it's in a whole bunch of protocols, including TLS, but depends what one includes in "binary format for structured data". ASN.1 is not self-describing, both sides need to pre-agree on the schema.
And then there's bencode, powering the protocol driving most of the data use on many parts of internet.
The format people should probably be using if they didn't have any particular requirements is CBOR, because it's an RFC. The awesomest one is probably Cap'n'Proto, because it's the Second System by a person who helped make Protobuf and learned from its mistakes.
Of course some people use netstrings, because they're a DJB specification.