I'm not sure why people keep asking the question... who is the most powerful mutant when we all know what the answer is!
I mean, Xavier can stop any mind at will but he can't do it if he gets struck by lightning by storm, or fried to death by Pyro. And forget rogue who can steal any power at will just by a simple touch.
Every mutant has powers over everything.
If that question is asked to me i would say they are all powerful but my favourite one and if she was up in a fight she would definately win would be Pheonix. Besides having kool fire control, she's also a telekinetic and a mind reader. In X-men 1 she stopped frog from attacking her, in x-men 2 she stopped Cyclops blasts from getting her, which his blasts are extremely strong. She levitated the entire jet as well as controlling everyone's minds in the jet including night clawler and xavier. She can put mental blocks so knowone can read her mind and in the cartoon series of X-men evolusion, she could create a telekinetic barrier around herself and stop Storms lightning bolt coming towards her. So, in a fight, she could block Xaviers telepathic power - give everyone mind numbing mental blocks and bolts (which her power is like Xaviers) - Stop anyone coming towards her and throw them through the air as well as stop anyone's powers from be used by controlling their minds.
IS MY QUESTION ANSWERED YET......