Take a look at the solution at:
On this page there is a link to a Visual Studio 2012 project you can download which provides offline mapping support for Windows 8 - but since it uses Phonegap, it can also target Windows 8 phone, iOS, android using phonegap build.
Given the TOUs for the various content providers, OSM is a good start. I'd also look at MapQuest. You would most likely be in violation of Google's and Bing's TOUs taking their content offline with the above approach.