I would say... Rob Lucci. Zoro really hasn't had enough time to level up much, mainly he's just gotten the ability to do Asura at will and gotten a stronger sword.
That's not enough to catch up. It'll be a fight, but Zoro's only a little stronger than Kaku.
well, I thought that zoro might have had a chance against him even during that arc, simply because his fighting style seems to be more of an advantage against lucci's. also, i always thought of zoro of being relatively close to luffys strength (relative, meaning luffy is insane compared to the rest of the crew, and zoro is higher than the rest) and he is ALWAYS working on something, weather it is just working out, or some new tech. however, from what has been "shown" he is not strong enough yet to take him on. but i think that it would also be kinda close, zoro would injure him, and lucci would resort to changing into panther hybrid form
Lucci has most of the advantages Kaku did (iron body, flexibility, speed), and Kaku was a darn tough foe.
He'd certainly have to go panther, but he's got the chops. Zoro's definitely the second strongest of the Strawhats without much doubt and could currently probably beat no-gear Luffy if he went all out, but Asura isn't quite as powerful as the Gears nor is Zoro quite as tough.
I'd think worst-case, Lucci's 7th and ultimate technique would end things.