Edward Weston, for his landscape detail pictures, still-lifes, and luminous shadows and smoothly graded greys. I had the pleasure of hanging an exhibition of his prints in undergrad, which gave me time to study them off the wall and without being crowded. Inspiring in his elevation of quiet subject matter, and excellent technique with minimalist tools.
I would add, of course, St. Ansel, though while impressed with his perfect landscapes the monumental nature puts me off a bit. This is probably similar to a difference in taste between 18th century baroque chamber music and 19th century orchestral drama. He had an image in the recent show of moon over some rounded rocks in Joshua Tree Nat'l Monument which was very dark, but the tonality and detail in the blacks drew me back repeatedly to the image, though it was less monumental than the Yosemite Valley ones. Wynn Bullock's images, and their use of time and decay, beautifully printed with lots of detailed dark are also appreciated.
In Portriature, I would say Halsman and Arbus, with the observation that different subjects deserve different photographers. Many a wedding album would be enlivened by a disciple of Arbus.