Also, it seems that I was right when I posted this back in July of last year.
My guess is on Kanjuro. He was captured by the DD Pirates and left relatively unharmed in captivity. Not to mention he is one of the 3 samurais to potentially have a vivre card to Zou. However it is interesting to note that the plan had been spilled after the Straw Hats laid it out.
Personally, he is the most suspicious.
EDIT: Let me extrapolate further. Kinemon explains that they all were separate, but they knew Raizo was going towards Zou. They might have gotten cards for Raizo so that they could reconvene with him, considering Zou is a constantly mobile island. Kinemon got separated with Kanjuro and ended up on Punk Hard. Also note that they could find Zou based off a vivre card. A vivre card belonging to Raizo. And the attack on Zou happened days before the Swirly Hats arrived on Zou. That's my conclusion anyways.
To think people honestly thought the traitor would be Carrot. lol
–- Update From New Post Merge ---
"Best chance" is not necessarily "good chance" and I don't even agree that Carrot has the "best chance". That would be Vivi.
Carrot wasn't given the attention that the rest of the crew got in their introduction arc so if she is meant to join that would be a huge letdown.
Vivi is an "honorary" member. Besides, Vivi said it best. She can't abandon Alabasta because as much as she liked sailing with the Straw Hats, she loves her country even more.
I said she has the best chance out of any of the candidates post-timeskip and I still stand by that.