Recently launched Responsive Select Menu plugin automatically turns your WordPress menu to a responsive select box / dropdown menu in mobile devices. It takes less screen, provides easier navigation for touch screens and works automatically; means there is no need of adding any PHP code in header like other plugins require.
Responsive Select Menu
Install and activate it then visit Appearance -> Responsive Select Menu and make customizations. You can select locations for applying menu, set your own width break point, configure navigation levels, set the test for first menu item and more, see the settings page screenshot.