When I worked for Southwest Airlines Vacations, the most expensive was San Diego because so many people took the cars to Mexico and got them stolen. Next was San Francisco. Not only was renting expensive you had to pay to park everywhere you went (including your hotel, which usually started at $50 per night). This was pre-Katrina, but we did not even offer car rentals in New Orleans. Theft and vandalism rates were way too high. So there's my top three.