Mystique

Why in the world is Mystique more powerful than Beast? Easy – she can shapeshift and she has the experience of several lifetimes. And, unlike Beast, her intelligence is not a purely theoretical one. So while Beast has PhDs over PhDs, she has gathered priceless experience on the field. Basically, she’s a super-spy who can read everyone at a simple glance – so no matter how athletic and strong Beast might be, she would still be able to anticipate his moves.

This being said, the fact is that Mystique is also outgunned when compared to other characters on the list. Of course, she can shapeshift – but without copying the abilities of her enemies. So she might be able to trick some at first – but how long will she able to do it?

As seen even in the movies, a mutant with superhuman senses could easily identify her (she did get an upgrade in the comics to counter that, but much later on).