This is quite interesting. I don't know if people will agree with me, but I will put those who I think should advance in the next round like this with futher explanations:
1. Vista. He is the best swordsman in WB's crew and he was the 4th commander of his crew and was as well mentioned to be a Haki user, even though not strong enough to stop or to damage Akainu. Still he was capable enough to stop Mihawk, even though we don't know how serious Mihawk was, but still even Mihawk himself praised Vista a bit.
2. Perona. I believe there won't be that many people to agree with me on this one, but I believe that unless we know more about Catarina Devon we might try to analise the possibilities in such a fight. Even though we know that Catarina was held in level 6th of Impel Dow, we still don't know how she fights, while we know that Perona's DF is really broken and if you aren't aware of its effect and don't attack fast and try to go for the KO, then you might loose to Perona even if you are quite strong...
I mean, how can we choose which one should win/loose when we don't know anything about some fighter (except maybe some hype made from their background, which, in the end, could mean nothing at all)
In the end I just feel that adding character that we didn't see fight is just transforming this contest into a popularity contest :-)
I agree that Catarina Devon can't be weak as she was in the 6th level of Impel Down, but still I do doubt that anyone conciderably strong can handle Perona's ability, especially when you don't know what it is and how does it work. And when you hit by it, then you will be damn useless.
I also doubt that now BB's crew is small. And even if it is relevantly small he still will have allied crews just like other Yonkous. And it was said that he collected different DF abilities and I doubt he only took them for himself. I believe he might have transfered them to his crewmates, but we don't know what did he do exactly. The same way we don't know if Perona's ability might be negated if you have a really strong will and Haki as Luffy has a damn strong will to never give up, but he was useless against Perona, the same way as Zoro.
I'm sure Catarina is a ferocious bish of a fighter but quite frankly we've barely see her in combat apart from stabbing a few knives and firing a few shots at WB when he was at his most vulnerable state. I don't give her any credit for that.
So for me it's Perona and Vista.
Vista stood his ground against mihawk which makes him the strongest in here for me