"Powerful" is a relative term. As the Chosen One, Anakin Skywalker possessed a higher quantity of midi-chlorians -- which were responsible for allowing a Force-user to physically access the Force -- than any other Force-sensitive being in history. However, after his incapacitation at the hands of his old Master, Obi-Wan Kenobi, his power was greatly reduced. His potential, prior to his defeat at the hands of Obi-Wan, was such that he would have been the most powerful Sith Lord ever and, as Palpatine says to Yoda, "Darth Vader will become more powerful than either of us!"