Add "Sort by discount" option to Sort Dropdown menu
Here's how to add a "Sort by discount" option to the sorting drop down menu.
Create a custom plugin
If you haven't done so already, create a custom plugin.
Add the following code to your custom plugin:
Save your changes.
More information about customizing the sorting menu can be found here: http://docs.woothemes.com/document/custom-sorting-options-ascdesc/