Vader is one of the most powerful sith, not THE that belongs to Sidious. It actually might be Vader > the 3 of them, but lore says otherwise being that Kun was still a force to be reckoned with by the time of Luke's jedi order. Granted he wasn't in a physical body but even so.
When I say the Most Powerful Being > The most Powerful Sith. Sorry, but I can't bring myself to not say that Sidious was the most Poweful Being in the galaxy at that point.