Kuzan, extreme diff.

Luffy would probably just bounce off the ice or something idk

Rubber doesnt do well in the cold

Kuzan high diff

Kuzan wins this. Now if you said Luffy can stay in this form constantly, I'd give it to Luffy, but Luffy becomes vulnerable since his awakening has a limit of some sort. And Aokiji has the stamina to fight against someone like Akainu who is likely yonko caliber for 10 whole days. The only way I see Luffy winning is if he tries to end the fight as soon as possible and even then, Kuzan is no easy opponent to face against, even for a yonko.

After what I've seen from Luffy fighting Kizaru in the manga, I have more than enough faith that Luffy in Gear 5 can handle someone like Kuzan now.