According to emperor Palpatine Darth Plagus was the most powerful Jedi. Palpatine actually asked Anakin to join forces with him to learn the secrets of Plagus and become as powerful as Plagus to get the power over life and death. Anakin was the chosen one but that was just to bring balance to the force not that he was the most powerful Jedi. It seems that the ancient Jedi where far more powerful out of the necessity that arose out of constant conflict. In the starwars future the power of the light and dark side was so imbalanced that one side had to hide from the other thus a physically powerful Jedi or Sith was not needed so much.
