Currently the most overrated photog is Mario Testino ---- bar none. Some underrated photogs include Bill King, Andrea Blanch, Sheila Metzner, and Denis Piel among other. I'm so glad this thread got started I found this website not too long ago, and this thread reminded me of it.
Underrated Masters of Photography:
http://www.popphoto.com/americanphotofeatures/3191/underrated-masters-of-photography.html
Underrated Masters of Photography:
http://www.popphoto.com/americanphotofeatures/3191/underrated-masters-of-photography.html