Shanks possesses Conqueror's Haki, but what sets him apart from all of the other Conqueror's Haki users?
For example, let's look at Doflamingo. Trafalgar Law says that Kaido could beat Doflamingo relatively easy. Before the timeskip, it was mentioned that Shanks escaped unscathed from a battle with Kaido, so logically:
Shanks>Kaido>Doflamingo or at least Shanks=Kaido>Doflamingo.
So why is Shanks, who possesses Conqueror's Haki and no devil fruit powers, considered so much stronger than someone like Doflamingo, who possesses both Conqueror's Haki and awakened devil fruit powers?
We know that Shanks is not the most powerful swordsman in the world (that would be Mihawk), so what is it about Shanks that makes him one of the four most powerful pirates in the world?