For years, the Toyota Prius has been topping the EPA mileage rating chart, most recently coming in at 50 mpg combined, with 51 in the city and 48 on the highway. (Perhaps this is true of the new 2010 Prius. My experience driving a 2006 Prius has seen a combined score more in the 47 to 48 range, which has involved much freeway driving.) Anyways, the Prius has been beaten by the Nissan Leaf getting an equivalent rating of 99 mpg. And the Chevy Volt was just announced -- 60 mpg "composite."
Here's the basic numbers on all three of these models:
Base MSRP Tax Incentive EPA Mileage Rating
2011 Nissan Leaf $32,780 $7,500 99
2011 Chevy Volt $41,000 $7,500 60
2010 Toyota Prius $21,400 0 50
So the dollar for dollar comparison looks like the Leaf competes most directly with the Prius, even though the incentive has expired on the Prius.