Definately toure due to him being unique. You don't replace yaya like for like because no other player has his skill set. Those seasons when players just bounced of him when he was on a run were amazing, no one else can do this. Plus the important goals he scored for us. Still has the ability to control the right game even though he has lost a yard.
Next would be Sergio then silva. If silva didn't completely fall to pieces in front of goal it would have been a harder choice.