To be honest, they all have doubts over them. The only one who I think will be really safe would be, Alguersuari. So I voted him, Petrov and Ricciardo.
Petrov, well he's proven to be better than Senna, was matching Heidfeld, with 2 seats up for grabs at Lotus-Renault, the most likely I would say are Grosjean-Petrov.
Ricciardo, mainly because Red Bull are very keen to get him into F1 one way or another.
I would have liked to choose Buemi but...I am unsure. Would be great if he stayed been unlucky this season. But I think Ricciardo will take his place.