Haha, asking a Californian where's the best beach is just opening a can of worms. Granted, I'm in Australia now, but I was born and raised in Southern California. No beaches in California hold a torch to the beaches here, in Hawaii, or on the East coast, in terms of water temp, clarity of water, or sand quality. With that said, they are some of the most beautiful, especially as you move north up the coast. I lived in San Francisco for awhile, and in areas like Big Sur, you have amazing coastal bluffs and cliffs, falling down to the beaches. In Southern California, you've got miles and miles of beaches, and although not the best in terms of sand quality or water temp, California sunsets over the water are amazing, rivaled only by Hawaii (or Arizona for that matter, but that's another story altogether).
Hope this helps!