I think Roger/PB is a pedestal meant to be surpassed by the MC first. Its a treshold that will be rendered null and less special if more people are at that level.
I think Akainu is very close to Prime Garp if not at that level. I think Shanks with his Joyboy level Haki can be stronger then Akainu but it depends.
You can't put Kisui's underling as a HK tier martial general