I just want to point out here that anyone who has their own opinion that differs from that of George Lucas...
... well, of course, you are free to have your opinion. But I want to point out that, in any official sense, your opinion holds absolutely no value whatsoever, and is purely personal.
We are concerned here with an official franchise, not a fan-created one, or anything like that. Within that franchise, there are facts, Many are misty and hard to determine, which is why sometimes you have to examine the evidence to come up with the best conclusion that you can.
But if facts are STATED- by, say, GL, or as Lightsnake has also pointed out, the author of the stories in question... then that is the end of the matter. The fact has been stated, and then any other evidence has to be viewed and re-assessed in light of that fact, or possibily rejected altogether as incorrect- as has happened before in the EU, from Splinter onwards.
Hence... if GL has indeed said that Sidious is the strongest of all time, or the creator of the Sith Lords in question has stated that he did not intend them to be greater than Sidious... then that's it. Argument over.
Assuming these statements did indeed happen, then anyone who thinks otherwise is simply, in any official sense, wrong.