Depends on the weapon I use but with my most used weapon, the Gunlance it must be Diablos, with a Lance/Gunlance Diablos is easy mode:-p Most annoying must be Kirin, stupid pony running all over the place, pretty annoying with a Gunlance. But with SnS it is kind of reversed, Diablos is pretty challenging and Kirin....nope still annoying ....but way easier.