Zoro is strong but not the best Haki user in ... Despite having two mighty Devil Fruit abilities, those being the Yami Yami no Mi and the Gura Gura no Mi, they are not his only weapon.
Yami Yami no Mi is the strongest Logia ... and compete on an equal footing against the likes of Zoro. Undoubtedly, this is one of the greatest Zoans that fans have seen in One Piece.