In all likeliness, Blackbeard, but Mihawk has yet to fight seriously and Blackbeard has not been seen since Marineford. Blackbeard is bound be be even stronger since then but at the same time Mihawk is bound to be stronger than he's shown. It's speculation, but Blackbeard is probably stronger at this point.
blackbeard is more powerful, but he always talks too much, that has costed him a jet pistol, a poison hydra, a fire fist to the face and getting sliced by whitebeard. if mihawk goes all out on his first move he might be able to end it there.