We are already more than four years into the last 15 years of the manga though, with three/four more emperors and ending still to go. Kizaru is not one of the big bad guys despite him being a top tier character. It's Doflamingo<Kaidou/Big Mom/Akainu<Blackbeard.
What you're saying is also true, but I was making my list by categories:
Blackbeard probably is the strongest Emperor, and definitely he's the emperor Luffy wants to take down the most. And until BB is down Luffy can't claim his Pirate king title.
Akainu is now Fleet Admiral, so he is technically the strongest Marine officier. Obviously strenght is not the main criteria used to choose the person to put in charge of the military forces, but we know that the guy is also sharp minded and very determined to do anything for the sake of World Government.
He alone was enough to make Blackbeard run away with his double devil fruit powers and his upgraded crew.
Kizaru. Heh.
Until the last admiral is shown, I'm pleasing myself by branding Mr. Kizaru the nastiest of Admirals. To me, he's a lesser Akainu, only less powerful and blessed with unearthly dumbness. He's implied in a number of not-so-noble actions, and I think that no one will miss him the day he's absorbed by a solar panel. He's a big bad guy because he really needs a good spanking.
Doflamingo it's a special case.
He's strong. Not insanely strong, but still very strong. But his status as a "Big Bad Guy" comes from what he is and what he do.
He's clever, and he has some dangerous knowledge. He can blackmail the Tenryuubitos without consequences, he's radicating himself in the underground world, and he's also giving an insane amount of power to a Yonko believing that in the future he'll still be able to control him, despite his military force.
Not only that, to this day no one really knows what he's really trying to do.
Ok, he wants to destroy the Tenryuubitos and the world they created, and he does not care of the suffering this process can cause. But how he's going to do that, it's unknown.
He really wants to be the Pirate King? He's going to make Kaido clash with the World Government? Or he wants to be the only king of the entire world? No one knows.
I don't think this is the end for Doflamingo, it's only the first act.