Has to be Blackbeard, in my opinion. Shanks is far too diplomatic (or possibly outright involved with the WG), Kaido’s just a suicidal alcoholic who also wants to destroy the world, and Big Mom is the biggest narcissist out of all of them.
Teach dresses the part better than the rest, but beyond that, he’s the only Yonko who isn’t afraid to go after what he wants and take it by force. The others have been in a stalemate for years because of the similarities in power, but Blackbeard doesn’t respect any of that detente shit and will swindle whoever he needs to con to get his way.