I believe Darth Sidious was one of the greatest Sith Lords until he named himself emperor and indulged in all the luxuries he had available to him. He turned his back on the true way. In the end he wasn't a Great Sith Lord but a weak Dark Force user. He should have trained an acceptable apprentice as was the tradition started by Darth Bane. But then we wouldn't have the newly appointed Darth Caedus. Hail the fall of Emperor Palpatine!
