If there is a stretch that is immediately and unmistakably New York, it is the area that runs east of Fifth Avenue in the 40s and 50s. The great avenues of Madison, Park, Lexington and Third reach their richest heights as the skyscrapers line up in neck-cricking vistas, the streets choke with yellow cabs and office workers, and Con Edison vents belch steam from old heating systems. More than anything else, buildings define this part of town. Many house anonymous corporations and supply excitement to a skyline that was largely formed during the 1960s build-'em-high glass-box bonanza. Others, like the Sony Building and the Citicorp Center , don't play that game; and enough remains from the pre-box days to maintain variety. The commercial properties largely disappear as midtown slinks toward the East River, giving way to the quietly affluent residential Beekman and Sutton places as well as the unappealing mass of the United Nations complex , which anchors itself like a barnacle to the eastern edge of the city.