There really isn't an absolute standard of "the best" university or college. Each individual department has various strengths and weaknesses and each school is either a good or bad fit for a particular student.
All of the UC schools have strong programs in many fields, and all are in the midst of drastic budget cuts which may negatively impact student experiences.
In order to help people answer more specifically:
- Are you looking at graduate or undergraduate programs?
- In what field?
If someone is already living in LA or the Bay area, it would make sense to go to the nearest school. If the issue is in-state tuition, it is worth looking at all the UC schools to see which has the best program in a specific field. No one school is the top in every field.