Nadar, Karsh, Irving Penn, Avedon, Beaton.
I do like Platon, Dan Winters, Ben Watts, and Martin Schoeller, but they are pop editorial, and not in the same league as the A list of dead guys I listed above.
Abdul, I think the problem you will have with this post is that portraits are very much a matter of taste and the psychology of the viewer. as an example, I don't like Patrick Demachelier at all, fashion, portrait, whatever, but I can see the appeal.