Does anyone know for a fact who Gossip Girl is?

[Cassie]

I have seen every Gossip Girl episode there is, and have read many of the books and I was just wondering who all of you other Gossip Girl fanatics think she is! My bet is Georgina I mean she comes back quite a lot and is hated by almost all of them, I know there is stuff posted about her on there also but I think shes smart enough to do that so that she doesn't get caught, so I want you guys to tell me you ideas!