Magnetic fields, which is part of electromagnetism. He can directly influence one of the fundamental forces of the universe. Magneto is definitely one of the strongest mutants.
If he’d learn to do things besides bend and move metal, he’d be able to do almost anything depending on how acutely strong of a field he could make. With how field lines snap in the sun and make for huge discharges of charged particles, I think Magneto would be able to do a lot even without making hugely powerful fields if he’d learn some science.