A friend of mine brought the topic up and I honestly can't say who'd win. Normally I'd say Magneto but knowing Tony, he probably has a suit designed to negate Magneto's ability, and Doom has his Magic as well and I don't know what kind of effect that'd do to Magneto's powers.
They're fighting in the middle of New York
Magneto's attempting to prove the power of Mutants
Doom's attempting to take over
Stark's trying to stop both of them