Said to be and that quote has not be supported yet...although Lighstsnake will disagree.
Really there are 3 contenders here : Nihilus, Sidious and Ragnos.
All of you seem to note about Sidious being intelligent and i dony deny it, and Nihilus's ability, however your forgetting Ragnos was no slouch in strategy either, he kept the Ancient Sith by pitting opponents against himself and others, he isn't a Sadow or Kressh....
Personally, given that we have no idea whether the Ancients can block Nihilus or not, Nihilus drains everyone and comes out on top.
Decay, your biting off Lightsnake's argument which has yet to be proven, also your assuming everyone will leave Sidious alone, your assuming that he is able to stand and watch, honestly in an all out battle like this, its diffcult to tell the outcome, for all you know it can be a Dark Jedi Apprentice.
However like i said the possible 3 candiates are Sidious, Ragnos and Nihilus.