Blackbeard is the Yonko that will likely serve as the final antagonist. He is also the one with the endgame goal that is most dangerous towards the whole world. Not only is he the only Yonko that is actively pursuing the title of Pirate King, he is also the person that has made many personal enemies. Aside from trying to become Pirate King, Blackbeard is known as a generally evil person who is feeding his crew with Devil Fruit powers in order to control and dominate the seas.
Blackbeard is likely trying to become the ruler not only of the pirates, but the seas in general. This makes him one of the most feared people in the One Piece world, by both other pirates and the World Government. His mysterious abilities and cunning strategies allow him to work his way up, and right now he already holds the rank of Yonko. Whatever happens in the future arcs, Blackbeard is most likely the most dangerous person in the world of One Piece at this moment, and he is still gaining momentum.