I lived in LA for a few years back in the day. I have tried all the places you mentioned in your post. While I liek Red Robin I can hardly consider it one of the top picks for LA. I think you are spot on with Fatburger and even more important In-n-out-burger. The latter place is magical. I wish I could have them once a week. Damn I remember eating them on a Sunday afternoon in Inglewood enjoying the nice sun on my back and fresh fries and an animal-style double-double. Its currently about an 18 hours drive for me now to get to the closest In-N-Out. I wish they would look at expansion.