All scalar types other than strings (especially nonsense like datetimes). I'm already ingesting the configuration file as a string and parsing it into a language with actual types, e.g. Rust, and the act of parsing it into my defined configuration type will itself identify any problems with the config; having half-assed types like "int" in the config file format itself is not only useless, it's counterproductive because it requires people to ask questions like "okay, but what actually is the maximum range of integer values allowed to be contained in this type?". Just enforce UTF-8 encoded string values and let me take care of data validation.