Royal Dutch/Shell and Exxon Corp.may never be the toast of "green" activists, but they can take comfort from a new report ranking them tops in the oil industry when it comes to the environment. Royal Dutch/Shell and Exxon took first and second place respectively in a study of the environmental performance of 13 leading oil companies doing business in the United States. Third place belonged to Mobil Corp. , while Los Angeles-based Unocal Corp. finished last in the soon-to-be released report.