To answer the question.
I think you would be tied between PB/OB (PB is north of the 8 less parking OB south of the 8 better parking, with mission beach right in between. All great beaches) and Oceanside.
This is do to location, parking and faculties, sand - the whole nine yards, They are fantastic beaches and get a ton of traffic due to the events that they hold.
Bottom line all the beaches on the California coast are stealer and as everyone before me has said it really depends on what you are looking for.