I say Giratina. It has a lot of HP and defensive stats, not to mention freaking Shadow Force. Normal- and Fighting-type moves can't even touch it, which, coupled with its large and unsettling design, makes it seem like one of those demonic beings in the movies which just show up and start killing everybody, and nobody can do a darned thing about it. It has a pretty epic intro in Platinum, too, even though I detest the Distortion World.