First Gandalf by the Brothers Hildebrandt. As I've stated before, the Hildebrandt's were the first visual interpreters of Tolkien to me, and so always hold a special place for me.
Second Gandalf by Michael Kaluta. Kaluta is a comic book artist who contributed pictures to the 1994 Tolkien Calendar. Kaluta's Gandalf is a little more feathery and furry than he is usually conceived.
Third Gandalf by Ted Naismith. Ted Naismith produced many Tolkien Calendars as well as illustrations for The Silmarillion. Naismith's figures are always firmly part of their environments, which are characters themselves. His Gandalf's hat seems pretty tall and skinny.
Fourth Gandalf by Darrell K. Sweet. Sweet produced a series of pictures for Tolkien Calendars that provided the covers for Tolkien's works in the 80's. I've always liked Sweet's solid details, with every fold and ornamentation having it's own weight.
Fifth Gandalf by Alan Lee. Alan Lee has illustrated The Hobbit, The Lord of the Rings, and Tales From the Perilous Realm, as well as being one of the main production designers for Peter Jackson's LOTR movies. His characters seem to have what I think of as Celtic suggestiveness, a sort of misty impression that is still strongly evocative of character.