[Noxomanus]

I think the Valar on purpose made him seem the least of the five (smaller,and leaning on a staff) because they knew he was the wisest and most trustworthy.Sauron would obviously think the Valar would make the wisest the head of the order,so he thought of Saruman as the greatest Istar and concentrated on him.Gandalf would probably be the last he'd go after wich gave him the chance of doing the greatest part of the mission of the Istari,working behind the sceens at Saurons fall.It probably wasnt until he returned as Gandalf the White that he really got Saurons attention. Unfortunately for Sauron,Gandalfs plans were nearly realised then.