The first picture shows Gandalf by Tim Kirk. Tim Kirk produced one of the first Tolkien Calendars (1975) not illustrated with art by Tolkien or Pauline Baynes. As such I believe he set many precedents in Tolkien iconography to this day. His Gandalf towers over Bilbo, and recalls a famous picture of Odin by Georg Von Rosen.
The second Gandalf is from the 1977 Rankin/Bass production of The Hobbit. This design recalls the work of Arthur Rackham, and neatly solves the dilemma of drawing Gandalf as having, per Tolkien's description, "long bushy eyebrows that stuck out further than the brim of his shady hat" by doing away with the brim of the hat. (As an interesting side note, long eyebrows are a sign of wisdom in Oriental art. The influence of Oriental design, such as the use of round doors in both Hobbit and Japanese architecture, might be a subject worthy of further study.)
The third Gandalf is from Ralph Bakshi's incomplete 1978 animated epic, The Lord of the Rings. This Gandalf's over-all bluish color scheme reminds me of Disney's Merlin from The Sword in the Stone. Bakshi used a technique called rotoscoping; that is, he filmed the action with people in costumes and then drew over that to capture the motion. Strangely enough, they recorded the voice work first, then had models dressed in costumes (not the voice actors themselves in all cases) mime to the soundtrack.
The fourth Gandalf is by John Howe, and was produced for The 1991 J. R. R. Tolkien Calendar. This shows Gandalf, not in any of his famous poses such as meeting Bilbo or riding an Eagle or fighting the Balrog, but simply travelling through Middle-Earth as he did for hundreds of years, going about his business on foot and in all weathers, earning his name as the Grey Pilgrim. This picture struck a chord with many Tolkien fans, even before Peter Jackson selected it out as the model for Gandalf for his movies and gave a copy to Ian McKellan to use as inspiration.
Which of course Ian McKellan brilliantly did as the fifth Gandalf. I can understand how Sir Ian could prefer the outfit for Gandalf the White as less bushy, layered, and heavy, but I still like Gandalf the Grey better, just as I would have to say I like The Fellowship of the Ring better than the other two movies. This is just a matter of temperament: I like FotR for the same reasons some people don't like it as much, because it is slower and has more history and character exposition and less action. It is like McKellan's portrayal of Gandalf the Grey; a little fussy but more human and approachable.