I'm a big fan of old-school diners, luncheonettes, and other greasy spoon eateries, especially ones housed in Googie/Mid-Century Modern buildings. And LA may very well have the highest concentration of diners built in this particular architectural style.
Not all of these places serve great food, though. So I'd like to narrow the list to places that are worth a visit based on the food alone. And when I say "great food", I'm not necessarily looking for anything fancy or gourmet - just honest, straightforward American comfort food (e.g. homemade corned beef hash, pancakes, eggs, burgers, sandwiches) that's a clear notch or two above middling greasy spoon fare.
Although not necessary, bonus points for diners with Googie architecture and an unpretentious, salt-of-the-earth vibe. The most important thing (besides good food) is vintage decor of some sort, preferably both exterior and interior.
Anywhere in the Greater LA/OC/IE area is fine.