To be honest, I can't think of a metal band worth nominating as the worst ever, simply because most of the shit metal bands today are shit because they're so drastically mediocre.
In that event, I will go for Bullet for My Valentine. At first glance or listen you might just think "They're not doing anything special.", and you'd be right. However, combine that with music that is contains less variation than the blueprints for Hitler's Aryan race and you have a shit band. There are lots of metal bands that aren't exactly reinventing the wheel, but Bullet for My Valentine are driving a Ford Capri with busted tires, but painting it to make it look like a new car.
-AC