To make sure your website drop down menus are user-friendly across devices here's one important tip:

Make sure that when you have drop down menus that your top level link - if it's an actual page link - is accessible from the drop down menu as well.  The reason is that on smartphones and tablets you don't have a mouse like on desktops/laptops and there is no "hover" vs "click" function.  The user just may click on the menu item, and the drop down menu pops open.

Example: On our website, we have the Features tab on the top menu.  On your desktop/laptop you can click on this and it goes to the Features page.  On a smartphone/tablet, you click on the Features tab and it opens up the menu.  This drop down menus contains the tab "Features List" which also goes to this Features page!  So no traffic is lost based on what device the website visitor is using!