Oh, I'm not arguing that Palpatine is the strongest and most powerful Sith. I am simply saying that more sources cite him as the strongest than any other being. That is a fact. More sources label him as the strongest than Ragnos, Kun, Sadow, or Revan.
Ragnos is cited as being "the most powerful of the most powerful" in the TOTJ comic book. That can be inferred that he is the strongest, and it can be supported by the fact that SuperSith such as Naga Sadow feared him. However, on the other hand, it could be that Ragnos was the most powerful of the most powerful of that time. The quote does not specify, whereas Sidious has been labeled "strongest of all time".
Emphasis on all time.
Granted, there are numerous inconsistencies which prevent me from agreeing completely. But if the arguement were decided by sources, then Sidious would indeed be the strongest. It is possible that he is. Just not guarenteed.
I came to this thread to disprove Janus's statements. I think I did a good job doing it, but I didn't mean to argue that Sidious is the strongest Sith of all time.