One of the best views of the Alhambra is from the well known lookout point called Mirador de San Nicolas in the Albaicin district of Granada.
The views from this mirador are good all throughout the day, but especially at sunset. The mirador stands opposite the Alhambra on a hill in Albaicin, and also offers good view over the rest of the city and of the Sierra Nevada mountains (when they're not covered by cloud).
There are numerous routes to the mirador from Albaicin though it can be difficult to find without a map as the streets in the Albaicin are narrow and winding.