As far as I can see (could easily be proven wrong, but I checked long enough to satisfy me) no one in One Piece has been called 剣聖 / Kensei / "Sword Saint" yet. Not even Mihawk. He he's 世界最強の剣士 / Sekai Saikyou no Kenshi / World's Strongest Swordsman.
from wiki:
the "sei" is different in Gorosei and Kensei, but its a pun. 聖 is saint, sage, holy person, and the "sei" in Gorosei is 星 for star, planet, heavenly body.
BUT the "Saint" in the name "Saint Jay Garcia Saturn" is 聖. Extending that to all the gorosei, the sword gorosei is literally a 剣聖/Sword Saint.
Mihawk might be the World's Strongest Swordsman, but there's more than just this world, and there's a level above the peak of a "swordsman".