I've been to a lot of cities, but very briefly - just driving through or for a U2 concert, less than 24 hours. I really enjoy driving through Louisville because it looks just like Grand Rapids, just a few more skyscrapers. My favorite to drive through so far has been Chattanooga, Tennessee. You weave in and out of these mountains and then all the sudden there's this city down in a valley. I've always thought it was very pretty and surreal.
Of cities I've actually spent a lot of time in, I like Chicago and Gulf Shores. NYC is great, but I think I like Chicago better because it's smaller and I can walk to anywhere I want to go (like from the museums and aquarium through Grant Park to Millenium Park and to the shopping areas). Gulf Shores is not a city, more like a strip of houses on stilts on a penninsula, it's fabulous.
Cities I've heard are great and would like to visit:
Tampa, Fl
St. Augustine, Fl (oldest city in the USA)
Washington, D.C.
San Diego