I would have to say that the x method is the best but pea is the easiest.
I have been told that that the X method provides less than ideal results. I then proceeded to use my google-fu to find out which application method works best for different types of coolers. Here is one of the best TIM application guides I could find which comes from benchmark reviews. They have a wealth of information and I consider their reviews to be of the same calibre as HWC reviews. They say a picture is worth a thousand words, so here they are.
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=170&Itemid=1