YAML and JSON are good ways to store data structures in. YAML, as by its acronym suggests, is a bit too "loosey goosey" for me when it comes to data grouping, as it comes very close to just creating your own simple formatting scheme.
External files with language parsing abilities have the advantage of testing new data without re-compiling source files. It gets better if it can be implemented in a way that it can save and load an edited file in run-time, so you don't have to restart your game session.