Out of all of the beaches that I have been to so far in San Francisco (disclaimer, I have only been to 3), Baker Beach has been my favorite. While the sun is more of a guarantee at Stinson Beach and the surfing is better at Pacifica, the view at Baker is unbelievable.
When I first arrived at Baker, I almost left because the parking lot was thick with fog and it looked like there was no hope for sunshine. Fortunately, the fog quickly rolled out, leaving us with a postcard view of the Golden Gate Bridge.
I definitely plan on going back, but next time I will bring not only a bathing suit, but also a parka. It's SF. Who knows what the weather will be like.