The bomb design is my favorite, but transferred over to a t-shirt I think I would have to pick one of the first two. I really like the animal skull in general though, both on and off the shirt. It has nice contrast and balance and the shape draws the eye.
I especially like the last design and the pyramid. the placement of the crossed hatchets I think throws off that particular shirt's likability, though, for me. These designs are all very pleasing, but i usually think, 'the weirder, the better.' In a tasteful way, of course.