Seriously doubt Gaara being the strongest either way he did loose to Naruto. I think so far it would be Itachi and I think his mangekyou sharingan would work on Gaara, and it would be easy for him to torture Gaara because all he has to do is make him see his past and pretty sure Itachi would take on Gaara's demon as well, he is extremely fast and all you have to do is hit Gaara to wake him up if that happened.