The X-Men's Storm has never tapped into her true potential - as a Sorcerer Supreme. One of the X-Men's greatest leaders, and also currently regent of the Solar System, Storm is not to be underestimated. She is an Omega-level mutant, defined as a mutant "whose dominant power is deemed to register -- or reach -- an undefinable upper limit of that power's specific classification." She has trained in the use of her powers to their highest level, giving her an edge in combat against even Omegas.
There's always been a mystical element to Storm's power. She was worshiped as a goddess by the Kenyan tribe she protected when she was young, and Black Panther has suggested there's good reason for their worship. He has argued there is a spiritual side to her power, in that she is super-charged by the faith of herself and others. Drawing on the faith of the Wakandans, Storm proved able to defeat a villain known as the Adversary - a sorcerous being so powerful it once took the sacrifice of an entire team of X-Men to beat him.
Storm's Mystical Potential Has Never Been Tapped
Storm is more than a mere goddess, though; she also has the potential to be a powerful sorceress, potentially even a Sorcerer Supreme. This can be seen in Uncanny X-Men #160, by Chris Claremont and Brent Anderson, when the X-Men were stranded in the mystical dimension of Limbo. The laws of time and space operate differently in Limbo, and the X-Men found themselves allied with a version of Storm who had become a powerful magic-wielder. This was reinforced by the Magik: Illyana & Storm miniseries, by Claremont and John Buscema, which revealed this variant of Storm had been stranded in Limbo and trained in magic by the demonic Belasco. He unlocked a potential that lay within her - and the miniseries subtly hinted at the scale of Storm's power, because she became the only person to ever break free of his corrupting influence, establishing a "Sanctum" - a term clearly associated with Doctor Strange, who has his own Sanctum Sanctorum.
Storm is descended from an ancient sorceress named Ashake, who had been blessed with sorcerous power by Oshtur of the Vishanti. This blessing was passed on through countless generations, with many of Storm's other ancestors wielding magic - perhaps even serving as Sorcerer Supreme. It is important to remember that the Vishanti were the ones who created the office of the Sorcerer Supreme, and such Masters of the Mystic Arts traditionally draw power from them - including Doctor Strange, who has served as their champion.
Unfortunately, outside of Limbo, Storm has never tapped into her magical powers. Uncanny X-Men #160 and the Magik: Illyana & Storm miniseries offer a glimpse of a road not traveled, one in which she pursued a different avenue of power to her mutant abilities. It seems likely she will continue to make this choice, with those magical powers abandoned.