If you mean the ones we have seen a display of their haki Vergo would be in contention for armament haki. I personally think Sabo would take that crown though.
If you mean haki in general Luffy seems to be quite adept at all three types and is steadily improving.
If you mean characters that have been seen then Shanks and Rayleigh are more than likely the characters that are near the top. Ntm the other yonko and the admirals. Dragon could be up there considering Sabo's is so good.