First Gandalf: by Robert Chronister, from the 1980 J. R. R. Tolkien Calendar. I always liked the style of this one: it reminds me of old German-style beer ads when I was a little kid. It's Gandalf busting Sam eavesdropping; technically the window should be round.
Second Gandalf: by Maurice Sendak. Once more from Sendak's speculative project of illustrating The Hobbit. Rather strange dimensions, but then it's something in the nature of a trial run.
Third Gandalf: by an unknown artist, from the cover of Tolkien and the Critics.
Fourth Gandalf: from the Sierra Entertainment game The Hobbit. This is the clearest picture I could grab off the internet.
Fifth Gandalf: by Carl Lundgren, from the 1980 J. R. R. Tolkien Calendar. A back shot, and the robes are way too long for him to actually stand or move around in them. It's a detail of Gandalf confronting Saruman at Orthanc. Here I think the folds of his robes are rendered in too great detail, especially in comparison to the rest of the picture. I wish Lundgren had produced another picture from Tolkien; I love his work.