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