When it comes to the ultimate offensive devices, sheer power is always required. The strongest anime arms and devices pack enough power to not only defeat enemies with ease, but potentially doom everything around them - including those wielding them.
Some devices, like the Bustermarm of Fairy Tail, are impossibly big and can cut through anything. Others, like the Punisher from Trigun, pack incredible power. Then there are the likes of Ea from the Fate series, which literally created - and can end - Earth itself. If you ever find yourself facing a device like these, you'd better hope your opponent doesn't know how to use it.