TheCarConnection.com submits: The 2010 Toyota Prius is again the most fuel-efficient performer in the U.S. market, at 50 mpg Combined, while runners-up include the 2010 Honda Civic Hybrid (42 mpg), Honda Insight (41), Ford Fusion Hybrid (39), and Mercury Milan Hybrid (39). That's the official word from the 2010 EPA Fuel Economy Guide information, which was released this past week. Complete…