In my experience, if you find that the wrote editor, that is the non-default editor, is opening when double-clicking on a file within the Solution Explorer then something is wrong with the underlying project's User Options file (.user) or the solution's User Options file (.suo). (I am not sure which, but I suspect the settings are stored in the .suo file.) Deleting the the .suo and all project .user files solved the problem.
I personally, set the Form Editor as my default editor for forms at the beginning of a project. After the forms are stable and require less user-interface design changes, I switch the default editor.