I have to agree sure any mage can be stabbed in the back, but as far as shear firepower they can call meteorites from the heavens to smash into your gathering army of warriors nothing else has that amount of impact on a battle. (in terms of realistic fantasy) A case can also be made for necros especially in siege type warfare. Disease has been the deciding factor in thousands of battles, whole cities wiped out in weeks. And in a large battle they have a constant resource of fresh corpses. But then a case can be made for all classes that can make poison.
In a 1v1 I would definitely go Mesmer as they have that whole tricksy gonna dominate your mind and make you see shit thing going on.
It really would depend heavily on situation any child with a gun can kill the toughest man on earth.