New York — Wal-Mart Stores Inc., the discounter that has become the dominant force in American retailing, is now the largest company in the U.S. and the world, capturing the top spot on the annual Fortune 500 list. Wal-Mart, No. 2 on the list a year ago, traded places with oil giant Exxon Mobil Corp. in the rankings compiled on the basis of companies' annual revenue figures. The retailer's ascendancy was expected after both companies issued their 2001 results earlier this year. The list of America's 500 biggest companies, published in the issue of Fortune magazine that reaches newsstands Monday,...