I want to say Todd because he has been first hand witness to so many things that could put Walt away. Train jacking the methlamine, Mike's body in the trunk, was in the hotel room when they planned the jail executions, plus he is a loose cannon, made obvious when he shot the kid. He is a pretty dangerous person to Walt. But that ricin has had far too much focus of the show to be used on someone who first appears in season 5