If you want your application to be cross platform, GtkLibrary is presently the best choice.

If you want to target Windows only, there is WindowsPresentationFoundation and WinForms.

If you want to target Mac only, there is MonoMac.

Other cross platform GUIs include:
 * [https://github.com/mono/xwt Xwt]
 * [WxWidgetsPort wxWidgets/wx.NET]
 * [http://cobra-language.com/how-to/Qyoto/ QT/Qyoto].

See also: DotNetProjects, LibraryTopics