I can only provide reliable information regarding San Francisco and Marin County. Using the 2 links below, you may sample properties above and below your specified price:
San Francisco $7M to $20M - http://bit.ly/SF7M-20M
Marin and North Bay $5M to $20M - http://bit.ly/Marin7M-20M
As far as the "most exclusive" areas, while others have indicated general neighborhoods, I would need to show you specific pockets and levels of exclusivity in each area.
Paragon Real Estate Group DRE 01844627