Cryptographic Message Syntax (a.k.a PKCS #7) is a very commonly used format for signed data. It also supports encryption and authentication of payload contents, so it's a bit of an uber-format. The downside is the complexity of implementation - the specification can be tiresome to trawl through and you have to be comfortable working with ASN.1.
OpenPGP format is likely to be simpler to implement and more readable to the human eye (no ASN.1 to be seen). This might be the best bet for simplicity. Again, you would have the option to add encryption at a later date, if you so desired. However, the specification can be equally infuriating to work with - I've never encountered an RFC that was so... imprecise before.