It can be argued with some confidence that the most urban intersection in Seattle is Broadway and East John Street, particularly at rush hour, when big-city crowds flow up the long escalators and out of Capitol Hill Station's main section. Once on the street, these crowds are at the heart of one of the densest neighborhoods on the West Coast. Across the street from the station is Nacho Borracho, whose facade is decorated with neon advertisements for Mexican beers. Once inside, you are greeted by festoons of colorful lights and banners. Behind the bar are machines that turn slushy, luridly lit booze. For a long time, those slushy machines have contained an avocado margarita that cannot be beat. Also, some of the best bartenders in the city work here. Each has excellent taste in music. CHARLES MUDEDE