4. Captain Rocks
Rocks Pirates used to rule over all the seas alone, unlike Roger’s era which had 3 pirates controlling the seas and right now there are four. This crew managed to do all this on their own. And it had crewmembers like Kaido, Big Mom and maybe even Whitebeard. So, in order to control them one has to be even stronger and thus Captain of Rocks is easily well above the level of Yonko or stronger.
All the Gorosei will easily be stronger than all Yonko. We haven’t seen them fight but it seems that they are the direct subordinates of Im-Sama and who knows maybe they are immortal. They are in my opinion the strongest force to exist and Yonko are will beneath their level. That’s just my opinion though. You don’t have to believe me. But that fact that they’ve performed several cleansings before implies that they’re definitely the strongest in the entire series.