Coming in 15th place is the Pacific Coast city of San Diego, which notched an average of 331 photographs per square mile. The metropolis often ranks among the most beautiful cities in America, and indeed the world, thanks to its golden beaches flanked by swaying palms, glistening skyscrapers and picturesque parks. Stunning streets include Mission Boulevard, a waterside boardwalk favoured by runners, cyclists and roller-bladers, and 5th Avenue, which is illuminated by 19th-century lamp posts at night.