List categories by post count — WordPress

functions.php

<?php
/**
 * Reorders get_categories() to list categories by
 * the most posts to least.
 * 
 * Example use: listing the most popular categories
 * on your blog. print_r() function to see results.
 */

function kol_most_active_categories( $args = null ) {
	$cats = get_categories( $args );

	foreach ( $cats as $cat => $fields ) {
		$count[$cat] = $fields->count;
	}

	array_multisort( $count, SORT_DESC, $cats );

	return $cats;
}