Huntington Beach is definately the easiest and most tourist friendly. If you're planning a quick trip to the beach from DLR, this is really the best bet.
If you've got a little more time and want to experience a more local flavor, then you can head north to my personal favoriate, Seal Beach. It is more out of the way (about 30-45 minutes to get to the pier and park) and more rustic, but it is still well kept and has plenty to offer.
I'm really referring to the pier area for both locations by the way. If you just want to go to the beach itself and swim in the ocean, then anywhere along Pacific Coast Highway between Huntington Beach and Seal Beach is great.