My favorite place to watch the sun set at the Grand Canyon South Rim is at Desert View. At this view point you can look east over the Painted Desert over the Navajo Indian Reserve, or West to see the sun set spectacularly through the Canyons.
In my opinion it's the best place to see it in Arizona, but I am sure other people have their preferences!
This particular view, over the Painted Desert, gives you a great view of the very east end of the Grand Canyon and the greatest view of the Colorado River from the top of the rim. Again, just like my prior photos of this place, once the sun starts to set, the walls of the Canyon are illuminated with all these fabulous colors.
This photo is taken just as the sun had set and I left the shutter open for a around 30 seconds to bring in the reds from the sky, and you can just see them creeping in from the left of the shot. At this point, it had gotten a little darker but there was still enough light to make the shot work for me. Also it's great at this point as most tourists leave as soon as the sun passes under the horizon so you are left with a very peaceful scene. Usually it will be just you and other photographers there...
This was shot with my Canon 5D mark II, EF 24-70mm F/2.8L and a 0.9 Graduated ND filter.