As someone who doesn't actually enjoy the "beach" that much, at least not actually swimming in it, I have to say Venice Beach is my favorite, due to the freakshow
Id have to say Manhattan beach. Although it sure has changed over the last 10 years from when I used to live down there. I must say they have a lot to offer for everyone from kids to surfers. The night seen is not so hot but if you just drive down the street a way to Hermosa its much better.