Metal, just think of the fun you could have with it.
I'd be able to control the calcium in peoples bone's and make them slap themselves and walk funny.
Also, by being able to control the iron in people's blood, I'd be able to cause an instant Viagra effect on blokes at the most inappropriate times, and make people pass out or just get amusingly dizzy by moving the blood to and from their head.
I have to say, I think Magneto would have been a lot happier if he just used his powers for the lulz rather than try to take over the world.