I'll chime in at this point...
If I'm taking portraits, my favorite time is nightime, when the people I'm photographing are more relaxed with the way they look. It just seems to work better than morning or daylight hours. I try not to use flash so a fast lens is preferable. Always a kick showing people their photos after a shot and they're amazed to see the results despite the absence of flash.
I tend to go into my stranger shoots 'cold' and usually warm up after engaging someone in conversation for a brief period. Always nice to put them at ease as to my intentions.
I don't make use of the early morning light often enough but I've seen so many beautiful pictures when the dew, mist or haze is still in the air, before being burned off by the sunlight. It can only enhance a photo in most cases.
Sunset and just afterwards is the perfect time to capture long shadows and soft light that makes the colors seem so deep and rich. Ansel Adams "Moonrise Over Hernandez" (black & white) is a fine example of this:
http://www.fada.com/view_image.html?image_no=3543
Those that are familiar with this very famous photograph might find this excerpt from Ansel Adams' biographer and assistant, Mary Street Alinder, interesting reading:
___________________
From "Ansel Adams: Some Thoughts About Ansel And About Moonrise", by Mary Street Alinder (Copyright 1999 Alinder Gallery)
"Moonrise was made on a typical Ansel trip to the Southwest in the fall of 1941 combining two commercial assignments: one for the U.S. Department of the Interior at Carlsbad Caverns and the other for the U.S. Potash Company. Accompanying Ansel were his son, Michael, and his good friend, Cedric Wright. The trip was a grand, meandering one, tailored to show eight year old Michael the sights of the Southwest. After a few days exploring Death Valley, the Grand Canyon and Canyon de Chelly, they decided to photograph about Santa Fe.
"Driving back to their hotel following an unsuccessful day of picture making in the Chama Valley, Ansel glanced to his left and saw a fantastic event. The sky was illuminated by brightly-lit clouds in the east and the white crosses in the cemetery of the old adobe church seemed to glow from within. He nearly crashed the car as he screeched to a halt in the roadside ditch, dashed out, yelling at Michael and Cedric to find the tripod, the camera, the meter, etc.
"Ansel rushed to assemble and mount the 23.5 inch component of his Cooke Series XV lens on his 8 x 10-inch view camera loaded with Ansco Isopan film and find the Wratten G filter. All was in place, but he could not find his Weston light meter. He remembered that the moon reflects 250 foot candles and he based his exposure upon that fact. He quickly computed a setting of 1/60 at f/8, but with the addition of the filter it became 1/20 at f/8. To achieve the same exposure with greater depth of field he stopped the lens to f/32 and released the shutter for one second. He prepared to make a second exposure for insurance. Dramatically, the light faded forever from the foreground.
"Moonrise, the negative, was far from perfect. It took me two years to convince Ansel to make a 'straight' print of Moonrise. He printed it without his customary darkroom manipulation as a teaching tool to show the basic information contained within the negative. Comparing this print with a fine print, one is struck by the immense work and creativity necessary for Ansel to produce what he believed to be the best interpretation of the negative. His final, expressive print is not how the scene looked in reality, but rather how it felt to him emotionally.
"Moonrise was Ansel's most difficult negative of all to print. Though he kept careful records of darkroom information on Moonrise, each time he set up the negative, he would again establish the procedure for this particular batch of prints because papers and chemicals were always variables not constants. After determining the general exposure for the print, he gave local exposure to specific areas. Using simple pieces of cardboard, Ansel would painstakingly burn in (darken with additional light from the enlarger) the sky, which was really quite pale with streaks of cloud throughout. He was careful to hold back a bit on the moon. The mid-ground was dodged (light withheld), though the crosses have been subtly burned in. This process took Ansel more than two minutes per print of intricate burning and dodging. Ansel created Moonrise with a night sky, a luminous moon and an extraordinary cloud bank that seems to reflect the moon's brilliance. Moonrise is sleight of hand. Moonrise is magic."