to Paradise! /eddie money
My favorite US cities (in no particular order): NYC, Chicago, San Fran, Portland.
Haven't been to San Diego, Boston, or New Orleans yet.
If you're considering Vegas and can get a rental car I'd recommend staying an hour outside of Vegas in Mesquite, NV. From there you can check out Vegas for a day or two but, better, you can take day trips to Bryce Canyon, Zion National Park, Red Rocks Canyon, and even the Grand Canyon.
NYC is Awesome - plenty to do: Broadway, Times Square, Ellis Island, Central Park, WTC, museums, delis, and on and on and on.
San Fran is also Awesome: Fisherman's Wharf, Golden Gate, Alcatraz, Mission District, Lombard Street, Muir Woods, Napa day-trip
It really depends upon what you're into: nature, art, theater, history, water/beach, entertainment