As much as I hate to say it, it has to be Wyatt Halliwell, Piper's eldest son. Not only does he possess the most number of powers of all the witches in the show, but he also developed many of his powers as an infant (some, even from the womb), but also shows a mastery of any new ability incredibly swift and has several offensive and defesive powers at his command. Plus, he possesses the two generally considered most powerful magical powers in the show's mythology: Molecular Combustion (the ability to blow up objects by speeding up their molecules) and Projection (the ability to make one's will or desire a reality, without the use of a spell or a potion).