The easiest one to start is XML.
The main reason for this is not parsing (for there are very good libs for XML and JSON that do this for you, see other posts), but the understandability:
JSON works with a lot of different parentheses, when looking at your XML data you will probably see any errors faster. (Assuming you know e.g. HTML)
It is also possible (but optional) to create an XML schema, that makes verifying your data automatically easy. This can save you a lot of time afterwards!