I said in my posts that I think mihawk can use haki just like marco etc.. but shanks has the emperor's disposition haki which make him more superior to mihawk in that sense.
I think that BB is the strongest and he will continue to get stronger like Luffy
Zoros ability is similar to the giants on the island and Mihawk and even when WB and shanks crossed swords like we found out on amazon lilly haki can be transposed onto ones weapons. From what we know about each member its hard to rank them in any particular order but I do know in order for BB to persuade pirates of such dangerous levels to join him he has to be somewhat of a bad ass. I see strength being determined as not only individual but ones allies as well. Mihawk is shown to be a loner which is why I cannot say he is stronger, GM lost most of his crew and what little weve seen of DF crew they arent much.
Personal strength is not the only thing that defines how strong one is. Heck in a one on one BB could not stand up to WB but with his crew backing him up he was able to defeat him the same goes for the SH they rely on eachother and give eachother strength.