Serializing isn't necessarily about performance, or we'd just use protobuf or similar. I agree Json is a great all rounder. Combine with JSON object schema to define sophisticated DSLs that are still readable, plain JSON. TOML is nice as a configuration language, but its main appeal (readability) suffers when applied to complex modeling tasks. XML is quite verbose and maybe takes the "custom DSL" idea a little too far. YAML is a mistake.