We are well aware of how cunning Doflamingo is, probably one of the best in the series. He had the connections (as it was implied), the people everywhere (Vergo), the authority (Shichibukai and royalty in Dressrosa). We have been surprised and were caught off guard more than once throughout the entire series about him and/or what he can do, with one of the recent being Punk Hazard arc (the emergence of Joker) and Dressrosa arc (the great resignation troll!). Not sure about you, I don't think it will be the last of such surprises we will get.
Having a person deep inside certain organization like Vergo is a really great achievement, not easy being able to penetrate so deep and plant a mole into the opposition's side. Reminds of a few great blockbuster movies from the East and West.
So here's where we speculate. Post PH arc, if the great man had another mole which has yet been revealed to us, who do you think is the traitor of the most shocking reaction from us (readers)?
Screw what we have seen or are aware of currently that may lead us to believe a certain person having the definity of NOT being the traitor. Like how we may think Momo not being a mole because we have seen his nightmarish dream / thought (do you remember that?). Quite easily Oda may be able to come up with a counter-story explaining the deal after the mole has been exposed (or perhaps he may not be bother to do so at all, like Vergo period).
So, besides the Strawhearts (Strawhats + Heart pirates) who will totally caught you off guard and made you wear the fault face like how Rici's avatar looks like? Momo? Kin? Violet (counter-betrayal)? One of the Colosseum participants? Rebecca? Barty? Another one of the ranked marines of recent exposure? Someone of the dwarves? TSOR?
For sure none can be the Strawhats, and for some reasons I am absolutely putting my bone-y foot down about Law and his Hearts (don't ask). Anyone else can qualify. Bellamy is now SEEN as going to betray Luffy, so he is an obvious choice but one that may not surprise many (if at all).
My traditional side poll: