August Sander, Paul Strand, Jock Sturges come immediately to mind
A bit of edge, but need to mention Helmut Newton. Love him or hate him, there is no mistaking the uniqueness of his style. In the age of complicated studio fashion and portrait shoots, he preferred one camera and natural light.
There are so many good ones, but check out the work of Jane Bown, I greatly admire her,and she works almost exclusively with just an Olympus OM1 and 85mm f2 lens.
Either way, the large Sieff book by Taschen is probably the best photo book bargain in the world, $25 for a huge book, beautifully printed, with hundreds of large, wonderful photographs!
How about Jean-Francois Jonvelle? Perhaps not all his photographs are portraits (or would be classified primarily as such), but they are beautiful.
My friend, coach and mentor, Monte Zucker:
Here is the basis of his portraiture: