You can create romance almost anywhere in the USA, but popular romantic destinations are New York City, San Francisco, Hawaii, Las Vegas, New Orleans, or Napa Valley.
Some of the many romantic areas you can make your proposal in is in Northern Arizona. The best place to m ake your proposal would be the unique town of Jerome. The architexture and the artistic quality of the environment gives Jerome a highly romantic vibe. Another good place to propose is Sedona Arizona.
I would suggest you take your love on a drive through the Pacific Coast Highway. It can start out as a road trip and end (either north or south) with a proposal! There are plenty of landmarks for you to propose.
although it kind of depends on what you like, i dont think there is much better then a golden sunset somewhere natural and beautiful...somewhere like yellowstone national park, thats if you are into nature though