Here is a fairly obvious secret: there is no bad sunrise or sunset place at the Grand Canyon. Equally obvious is that it won't be crowded anywhere for sunrise! The rest of this page refers to sunsets.
For sunset, you can get a gorgeous and crowded view at Hopi or Mohave Point on the free shuttle-bus route toward Hermit's Rest. Don't worry--the buses wait until well after sunset for the last returning crowds. If you get there a bit early you might have a chance to be in the front row along the railing. These are truly marvelous places to see the sunset.
Perhaps you will end the day at a viewpoint on the east rim drive toward Desert View. Many of these points have great sunsets and are less crowded.
For semi-solitude, try walking east from Yavapai or Mather Point. Start on the paved Rim Trail, walking to the right as you face the canyon. If you are unusually lucky, the setting sun might peek out below a ridge of black clouds. Bounding off a cliffside, a beam of brilliant light will paint the canyon and sky.
For complete solitude you can park at the Shoshone Point Road off the East Rim Drive. There are restrooms near the rim. Walk the dirt road 3/4 mile, then a short, optional and possibly scary path, to the rock that juts out over the rim. Take a flashlight for your return!
Here is another thing: the best sunset views are wherever you can see to the east as well as toward the west. Toward the east, away from the sun, is where the light glows on the cliffs and temples. Toward the west you are just looking at the sun itself and missing the last light on the beautiful canyon walls.
And finally, some of the best sunsets start an hour or two before sunset! That's when lengthening shadows splash an ever-changing pallete of light and moving shadow across the cliff walls.