NYC - It is simply the best. You can find any type of food you want at any hour of the day. It has Broadway, great museums, famous sports teams, landmarks and an energy that no other city in this country can match.
SF - it is beautiful, great seafood and sourdough bread. Love the hills of the city (Lombard Street) and the cable cars.
Miami - very simple answer, South Beach is wonderful!!
Seattle - beautiful city on the water with great views of Mount Rainier. Plenty of fun things to do like Pike Place Market, Chihuly Garden and Glass, Space Needle and cool music clubs.
Washington DC - so much history of the US with great museums/landmarks. It is also very easy to get around the city by walking or metro.