Well, first of I'd have to vote for Bill's city of residence and nominate Casino Point as the best Southern California shore dive (it's in the Long Beach Unified School District, so it's hardly a foreign country!).
But if you're going to stick to strictly mainland beaches, there are lots of places in Palos Verdes, Laguna and points south with very good diving. However, most of these involve a considerable schlep down a hill (sometimes goat trail, in the case of PV) from car to water. Those of us past the age range of personal indestructibility appreciate convenient access.
With that in mind, I'd nominate Leo Carrillo in Malibu. Short, horizontal hop from the parking lot, nice reefs and kelp. The only other place I know in L.A. or Orange County with as easy access is White Point in Palos Verdes, and despite the interesting angle of the warm-water vents there, the nearly-always-poor vis is a big drawback.
I don't know San Diego diving as well -- the Marine Room would rate as a reasonably accessible spot, and I'm sure there are others. But if there's any other place than Leo Carrillo in L.A. or O.C. with as good diving and less than 10 feet elevation change from car to water, let me know!