I feel obligated as a Californian to say In-N-Out, buuuut in all honesty, I think they're over rated. Don't get me wrong, they're the best fast food burger, but it's still just a fast food burger... It's good, I just don't think it's the second coming of Christ that a lot of California makes it out to be.
Sooo, best I'd have to say Schlotzsky's, which I'm so excited for because they're finally showing up in Northern California! (Grew up in another state and haven't seen one since I moved)!! I think they specialize mostly in sandwiches, however they have the best mini-pizzas, especially for something you can get through a drive-thru.
Worst? Well, this is biased, but Chick Fillet, because I can't eat anything there because they use peanut oil. So obviously, I've never really been a fan. : D