Being from South Florida, I would want to say Miami or Ft. Lauderdale, but that is not likely to be the correct answer.
While the question itself may be simple, I would suggest that the better question is which city is the hardest to maintain a good quality of life. What I mean is compared to income and other factors, which city is harder to live in.
The answer is San Francisco, CA overall, going by cost of living. If you go by capita statistics (avg wealth per person), then it's likely to be a smaller town full of millionaires like Lake Forest, IL or Jupiter Beach Island, FL.