It seems Kizaru ~ Kaido to me. Kaido just had the strongest in the world alias because of his roughness and brute strength. Still, when he was on G5 it was clear he was about even with Kaido, edging him out in the final blow. Similarly, Luffy spent all his strength to stop Kizaru landing that final hit, and we don't even know if that took Kizaru out for good. Nika Luffy took out Kaido upon wasting all his strength much in a similar way. It just seems like the admirals are tied up in terms of power and prowess with the Emperors, for more that the story seemed to show otherwise when the focus was on the Emperors, but now Kizaru forcing Luffy to his limits as Nika pretty much implies in one on one the admirals are as impressive as the Yonkos. Maybe the lead the Yonkos had as in hesitating the admirals to attack them was their vast crew, which they couldn't compensate with the marines, but having assistance from the world government with the now powered up versions of CP0 such as Lucci, it'd be a hell of a battle just as it's being now with the straw hats, with Zoro, still for the likes of it, having his hands full with Lucci.
I predict Zoro will take down Lucci, but it won't be easy. It will force him to his limits like King did more or less. Zoro said something like "if this is all this guy has, then it shouldn't fall to the captain to defeat him", which isn't the same as calling him weak, just not strong enough so Luffy would need worrying dealing with him. I dunno which battle Sanji will take, maybe vs Kaku, but I find that a little unlikely.
But yeah, as strange as it sounds, it seems like the admirals are up there to the Emperor's levels and Lucci and the CP0 are up there with Kaido's strongest subordinates. We all know that power ups go up immensely for anyone, just not the main cast, in shounens. I wouldn't like for Lucci to have become that strong but if Luffy did and Lucci was always seen as the gifted one from the CP9, having now achieved staff of CP0, it isn't that hard to swallow. Still tho, what was dumb, was Luffy using G5 to battle Lucci, but he was still whooping his ass at least. So Lucci is likely to be somewhat below G4 Luffy at his maximum when he was giving Kaido a good time, and just there at Zoro's and King's level, more like in between the Luffy that fell to Kaido when Kaido said his techniques were still lacking (when Luffy had learned the Haki part 2) and the Luffy who learned using Haki with Haoshoku and was entertaining Kaido... more like a
Luffy vs Kaido (when Luffy was KO'd 1 hit) : 1
Luffy (upon training the invisible shield Haki) : 3
Zoro/King/Lucci : 4
Luffy (learning Haoshoku in the attacks and with G4) : 5
Luffy Nika/Kaido/Kizaru : 7