MessagePack if you want a self describing format, it's pretty much json, but binary.

Watching this thread.

This is one of the most annoying decisions I have had to make for a nostr-ish thing I have been working on for ages. My final solution was to just make the first byte a version so I can pretend that it won't just be Json forever.

Even though I don't use it, I think gRPC?

CBOR

Application specific. If it needs to be self describing, CBOR.

I think the self-describing, binary format use cases are pretty few and far between, personally. gzipped or bzipped json is generally preferable in most cases.

Good fucking question

I like protobuf for the language support and tooling

thrift

Depends on what you are trying to do, and if and how you want the protocol to evolve in the future.

The first few bits are version bits. The rest is application dependant.