It's Kaido > Akainu confirmed by the author of this manga.
Kaido being able to beat Luffy, doesn't prove Akainu can do the same.
The difference between Kaido and Big mom is even closer, and it's obvious Big mom > Akainu too, but if Big mom can't put herself in a position of killing them, Akainu won't be.
Akainu's lethality only works against off-guard opponents or when marineford WB taking all the shots from even fodders marine bullets, swords and get no-diffed by Blackbeard pirates, and when Ace turns his back to save off-guard and already tired exhausted Luffy, similar to how Shanks lost his arm to a fodder beast while saving Luffy.
Akainu couldn't kill Ivankov, Inazuma, Curiel, and Jinbe, he only injured Jinbe later, when Jinbe put himself in a same position with Ace (turning his back to save unconscious Luffy).
Akainu and Ace clashed (when Ace was already wounded and tired), Ace didn't get ragdolled or KOed, he only get his hand burned, the clash distance was the same. It means that if Akainu only won via special DF advantage match-up, we've seen Ace could deflect Admiral attacks when he blocked Aokiji's.
So, act 1 Akainu vs. Luffy means Luffy will also get help from Law, because Akainu won't be able to finish Luffy as quickly as Kaido did, WCI Luffy + Law vs. Akainu is a close fight.
Law tried to help Luffy when it was too late, then he got shot by Hawkins's Kairoseki weapon, Akainu won't be able to do any of this, Hawkins wouldn't interfere.
Akainu loses in other scenarios.