One of my favorites is Iceman.
As a character, Bobby Drake is cocky and immature, but his arc has taken an interesting turn around the events of The House of M, when it came out that he was perhaps one of the most powerful mutants in the world, able to not just make temperatures be cold, but to actually have the ability to manipulate energy and matter itself, with ice being the "laziest" manifestation if this power.
But because of his lack of focus, discipline and drive, he has, his entire life, been unable to harness this power. Upon this discovery, he spiraled into a bit of depression, using the events after the House of M to pretend be didn't even have his mutant power any longer, just to avoid the pressure and stigma of not login up to the perceived potential of his power.
This, to me, speaks to so many of us as individuals who can be excellent at what we do, but still told we are not living up to the full potential others have of us and expectations other project onto our lives. Iceman was one of the most effective cryomorph mutants in Marvel, able to use his power defensively as armor, offensively as projectiles, functionally in terms of ice bridges for travel for himself and others... but because others found out that he had the power to do so much more, he suddenly became an underachiever and lost confidence in the abilities that came naturally to him.
Oh, and Psylocke is really hot, so I'll throw my lot in with others here as well on her.