List top 5 authors with most posts

Are you using WordPress 3.1+? There’s a nice get_users() function that’ll do the trick! However, you will need a little magic to boot:

add_action( 'pre_user_query', 'wpse_11832_pre_user_query' );

/**
 * Adds "post_count" to the SELECT clause. Without this, the "post_count"
 * property for users will be undefined.
 * 
 * @param object $wp_user_query
 */
function wpse_11832_pre_user_query( $wp_user_query ) {
    if ( $wp_user_query->query_vars['orderby'] == 'post_count' )
        $wp_user_query->query_fields .= ', post_count';
}

And example usage:

<?php foreach ( get_users( 'order=DESC&orderby=post_count&number=5' ) as $user ) : ?>

    <?php echo $user->display_name; ?> (<?php echo $user->post_count; ?> Posts)

<?php endforeach; ?>

Important: You must order by post_count, otherwise it will be undefined.