• Whatever is best for the data model and/or languages you use. JSON is a common modern choice, suitable for most things.
• If you want something more tabular, closer to CSV (which is a valid choice for bulk data), use strict RFC 4180 compliant data.
• If you want to specify your own binary super-compact data, use ASN.1. I am also given to understand that Protobuf is a popular modern choice.
If you aren’t in a position to choose your standards, just do whatever you need to do to parse whatever junk you are given, and emit as standards-compliant data as possible as output; again, RFC 4180 is a great way to standardize your own CSV output, as long as you stick to a subset which the receiving party can parse.
Nobody needs “USV”, and nobody should use it.