In 10 man, it would be something like -
Difficulty in terms of personal responsibility - Siegecrafter
Difficulty in terms of amount of ways to fuck up and die - Paragons (even though I would rate this fight below the other two)
Difficulty in terms of numbers - Garrosh
Garrosh would overall be harder than both of those by a mile. Siegecrafter is the most fun of the three fights though.