Julius Rudel, 85, is back conducting at New York City Opera for the first time in 25 years. “Collective memory is short. So not everyone may remember what Mr. Rudel stood for during his tenure. Under him, City Opera cultivated a couple of generations of American singers: Samuel Ramey, Carol Vaness, and, of course, Beverly Sills, long the company’s reigning diva and ultimately Mr. Rudel’s successor.”