People should realize by now that in One Piece, that with the amount of Top Tiers in the verse, no one person is going to have the absolute "best" form of any type of Haki with the exception of maybe Luffy with CoC, because not only is it a rare power to begin with, but probably even rarer for someone to actually specialize in it.
I'm sure there is a plateau in regards to both Armament Haki and Observation Haki that has been achieved by multiple characters. It all depends on their Haki affinity, the type they specialize in, and how much effort they put in to train to get there. In regards to Armament, I can see Garp definitely having the highlest level of Armament, and possibly Mihawk, but that's dependant on whether decides to make him an armament specialist, or an observation specialist. If he's an Armament specialist, then his will be at the highest level. Big Mom is probably another one who is at the highest level of Armament Haki as well, as it seems her invisible Haki is active 24/7. Kaido is another one who is probably at the highest level of CoA also, as he was able to one shot G4.
Right now it is hard to gauge who is where with what until Oda finally buckles down and explains the different sub-genres within the Haki categories.