I know that Everest is one of the least difficult as is Cho Oyu and I've got a feeling Broad Peak isnt that tough either (as far as climbing in the death zone goes!) and K2 is the hardest. Ive also heard that Kanchenjunga is quite a tricky one. Correct me if im wrong.
In reply to peteclark: thuis article on summit post gives an idea of the comparisons in difficulty in the paragraphs for each one that start about one third down the page
