Not entirely correct. While Darth Sidious is unquestionably powerful, and absolutely one of the most powerful to have ever existed, you could also easily make the argument that Marka Ragnos, Darth Bane, or even Revan were as powerful or perhaps even more powerful. The problem with this kind of dialogue is that the majority of Sith that qualify for being "most powerful" never exist in the same time frame. In other words, you can make the case that no single one of these Sith are the most powerful, and instead are simply the most powerful at the time of their existence.