E.O. Hoppe as far as studio portraiture goes...but I mostly admire his other types of pix.
I would say August Sander, but those aren't really portraits when you get down to it.
Most of my favorite portrait photographers were mainly photojournalists and/or documentarians, but I do like Hoppe's work in studio, so that's what I put here. He did much typology as well, and did it all over the world.