I remember George Lucas (GL) stating that Sidious was the strongest Sith, that Sidious desired to have Anakin as his apprentice because he had the potential to be 200% as powerful as the Emperor, but after he was injured in his duel with Obi Wan, he was only 80% as powerful as the Emperor, putting him on par with Maul or Dooku, which is still very powerful, but not what he had hoped for. I believe this interview is from the audio commentary from the EPIII DVD.