Californians don't have an accent?? Everyone has an accent...just because you're accustomed to it doesn't mean that it doesn't exist
This discussion should be titled "American Accent." The answer is southern Iowa, central Iowa, and parts of the adjacent regions in the states immediately to the east and west of Iowa. By no means is that a "prestige dialect," nor does it have many speakers, but it is the accent perceived as most neutral to the greatest number of Americans, which explains why it is the accent media personalities often attempt to acquire.
Philadelphia??? If you want to pronounce 'water' as 'wooder' then yes, you should go for that dialect.
Hepburn's accent was entirely fake.