Wordpress - Reset default roles and capabilities

If you don't want to install a plugin to just to do this one task, you can call the WordPress function directly to rebuild the roles and their capabilities.

if ( !function_exists( 'populate_roles' ) ) {
  require_once( ABSPATH . 'wp-admin/includes/schema.php' );
}

populate_roles();

I use User Role Editor. You can delete added roles, and reset roles from the plugin page.

To reset the administator role, you can, as of WP 3.4+, add

define('URE_SHOW_ADMIN_ROLE', 1);

to your wp-config.php file. After having done so, you need to go to Users > User Role Editor and click "Reset".