Well we all know it should be Ianto. He already knows more than any of them, and in particular he knows how
Jack thinks, and can access the archives. The knowledge that Tosh and Owen have is too specialised. Gwen will try to stick her snout in wanting the glory, but she is just not suited to the task. Ianto will just go on doing what he does, and it will dawn on the others that he is in fact in charge as he knows the whole system