WP_Query by MAX post_id?

try this

<?php
$new=new WP_Query(array('posts_per_page'=> 1,'order_by'=>'DESC'));
$max_id;
if($new->have_posts()){
while($new->have_posts())
{
     $new->the_post();
     $max_id=the_id();

}
  }
echo $max_id;