The Luffy vs Kizaru was resolving around who will stop the other from accomplishing his mission(saving / killing Vegapunk). It was not a conventional 1 on 1, and even then Kizaru failed until he received back up with Saturn's arrival
If you have Akainu above Kizaru, so is Kuzan. They were the fleet Admiral candidates, they were the ones with antagonistical dynamic and df powers, they were the ones who stalemated each other for days before Akainu barely winning, they are still the admirals most relevant to the plot right now(the only ones relevant to the quest for the One Piece).
And I'd even argue even Fujitora might end up above Kizaru.