DirectX version 11.2 is the newest version of DirectX available. It is being used on Win 8.1 if I'm not mistaken.
DirectX 11.3 may be what you are most interested in, which will be available at the same time DirectX 12 is, when Win 10 is released. DirectX 11.3 is more of a continuation of current DirectX API's, with new features, and may require new video cards to use all the latest features. DirectX 12 is a new low level API, much like Mantle.