WB was the only one capable to fight with Roger, the others were clearly below, Roger died and WB was left the obly one in that league. That is also why he was considered the ruler of the seas and no Kaido, BM etc. dared to challenge him.
Kaido has the strongest zoan and so the strongest creature (a dragon), also after WB was dead he had the highest bounty among pirates (still waiting for Dragon) and so he was labeled as the strongest pirate even by Kidd and the others.
Mihawk and Shanks are most likely, the same as Roger and WB swordwise. They were the clearly peak in sword skills, nobody was close enough to them. At a certain point Mihawk get the WSS title and is still waiting for a worth rigal to the point he is bored (and has also to teach himself potential rivals).