Although Chicago isn't in the top 4 or 5, being on this forum has informed be of it's strong points, and I now definitely think it's world class. I think it would be great if the city got some more national and international attention though.
Also, SF and Boston are some of the nicest cities of their size in the world. Not big cities, but very well layed out, beautiful, with good restaurants, shopping, street traffic and diversity. Frankly I can't think of another city in the world with an urban core their size that offers so much.
I don't know Washington too well, but figure it's also nice, with it's huge collection of museums, libraries and sheer power (number one in the world for that).
Miami is great, but like LA, I don't really know if I'd call it a city. Definitely world class though, whatever you want to call it, and kind of like a mini Hong Kong for Latin America (from a business, not skyline) standpoint.
Las Vegas is the world's premier gambling destination, and possibly America's biggest party place these days. Whether you like it or not, it's world class in its way.
I haven't been to Seattle, Portland, Minneapolis, Denver, New Orleans, but from what I've seen and heard of them, somehow don't think they make the cut to world-class. Seattle and Portland might be the closest, but in photos and diversity at least, are blown away by neighboring Vancouver. New Orleans seems like a glorified adult theme park, but then again I haven't been there.
After that, the list starts to thin out I think. Atlanta, Houston, Dallas, etc are more embarrassments than anything else. Try comparing Houston to Rome or Taipei and tell me that it's world class. The lifestyles are probably "world class" though, if you're into the American dream, with it's big houses, lots of space and land, swimming pools, brand-new cars, and in the sunbelt, palm trees. As I wrote somewhere else, lots/most people in the world would probably take a brand-new 400 square meter house on a big chunk of land, with a private swimming pool and two new SUV's parked out front in Houston over a small apartment in NY or London.