Magneto boasts that he could tear the world in two, and most of his enemies believe him, but even he pales in comparison to anyone who has been possessed by the Phoenix Force (e.g. Jean Grey). As you point out, Phoenix eats stars. Then again, the Phoenix Force is a cosmic entity, so a mutant augmented by the Phoenix Force is not at all the same thing as a normal mutant.
I would say Franklin Richards would be the most powerful mutant, although in the present he is a toddler and his full powers won't come until puberty.