10. Emma Frost
How did the former White Queen get to be such a, ahem, ruthless and powerful member of the X-Men? "World class breeding" according to Emma Frost herself but, like her other Omega Level peers, her abilities were Earth-shattering from day one. Throw in the secondary mutation where she can turn her skin into an impenetrable diamond-like substance, and you have a serious contender for one of the most powerful mutants currently serving on the X-Men. Even if she is a homewrecking hussy.
One of the team's many powerful telepaths (we guess Scott Summers has a think for 'em), there's been times when Emma manages to trump even Professor Xavier with her mental abilities. Like most mutants, her powers manifested in her teens, and were mainly limited to reading thoughts. As she got older and more disciplined, however, she found herself able to control the minds of not just one but hundreds of people at a time, to switch her brain with others, and able to cause people great mental anguish with a single touch.
Actually, the diamond skin thing is all the more impressive when you consider that it's not just like a layer of armour over Frost's skin, but an actual shift of her atomic structure so she is entirely made of an impenetrable substance. She has also been known to induce or boost other mutants' abilities by rewiring their neural pathways using her telepathic abilities. So, all around, pretty darn powerful.