I don't know what things are like in California, but there is a similar law here in Alberta and it is indeed enforced, i've known several young cyclists who have been ticketed because of it.

The best one to have is the one that you enjoy wearing the most. Go find ones you like the look of, and that are comfortable.

Personally, I also like the ones with a back of the head strap system, that system helps make the helmet a bit more secure, and can help reduce pressure points. I would make a personal recommendation to look at helmets that have this feature, many do.

But as SesameCrunch points out, the only critical indicator you really need is acknowledgment that the helmet is certified by your national standard.