United shares are up more than 80% since Jan. 1, more than double the gains of Delta Air Lines , the country's most profitable airline.United Airlines is the best-performing U.S. airline stock this year. It wants to be the most profitable carrier, too.United has been flying for nearly a century and is the world's largest airline by capacity. The Chicago-based carrier has been trying to expand its profits by investing heavily in roomier seats and growing a network that will excite high-spending travelers, wi ...