Default is technically "Strict Open XML" format aka .docx.
Office 2007 muddied the water by supporting a not so strict XML format, and the "Strict" Open XML format. As long as your using Microsoft software it's no issue.
Office 2013 also supports Open Document Format version 1.2 which is the default for Open/Libre Office, Google Docs.
Unless you are using fancy formatting options, can't go wrong with saving it in RTF / Rich Text format which just about in any word processor.
There's also the Microsoft Office Compatibility Pack that allows older versions of Office to open up the new format.