Here are is an incomplete list of some vehicle models that would make good hypermiling cars:

Ford Focus
Mercedes-Benz E320 Bluetec
Nissan Sentra
Pontiac G5
Cheverolet Aveo
Geo Prizm
Toyota Corolla
Honda Accord
Toyota Prius

Here are the most (and least) fuel efficient cars as ranked by the Federal Government at FuelEconomy.gov:

2008 Most and Least Fuel Efficient Vehicles (ranked by city mpg)
MOST EFFICIENT OVERALL		MPG
		City	Hwy
	Toyota Prius	48	45
Most and Least Fuel Efficient Cars (by EPA Size Class)		MPG
		City	Hwy
Most Efficient Two Seaters
	smart fortwo Convertible 3 cyl, 1 L, Automatic(S5), Premium	33	41
	smart fortwo Coupe 3 cyl, 1 L, Automatic(S5), Premium
	Mazda MX-5 4 cyl, 2 L, Manual(5), Premium	22	27
Most Efficient Minicompact Cars
	MINI Cooper 4 cyl, 1.6 L, Manual(6), Premium	28	37
	MINI Cooper 4 cyl, 1.6 L, Automatic(S6), Premium	26	34
Most Efficient Subcompact Cars
	Toyota Yaris 4 cyl, 1.5 L, Manual(5), Regular	29	36
	Toyota Yaris 4 cyl, 1.5 L, Automatic(4), Regular	29	35
Most Efficient Compact Cars
	Honda Civic Hybrid 4 cyl, 1.3 L, Automatic(CVT), HEV, Regular	40	45
	Toyota Corolla 4 cyl, 1.8 L, Manual(5), Regular	28	37
Most Efficient Midsize Cars
	Toyota Prius Hybrid 4 cyl, 1.5 L, Automatic(CVT), HEV, Regular	48	45
	Nissan Versa 4 cyl, 1.8 L, Manual(6), Regular	26	31
Most Efficient Large Cars
	Honda Accord 4 cyl, 2.4 L, Manual(5), Regular	22	31
	Honda Accord 4 cyl, 2.4 L, Automatic(4), Regular	21	31
Most Efficient Small Station Wagons
	Honda Fit 4 cyl, 1.5 L, Manual(5), Regular	28	34
	Honda Fit 4 cyl, 1.5 L, Automatic(5), Regular	27	34
Most Efficient Midsize Station Wagons
	Volkswagen Passat Wagon 4 cyl, 2 L, Manual(6), Premium	21	29
	Volkswagen Passat Wagon 4 cyl, 2 L, Automatic(S6), Premium	20	28
Least Fuel Efficient Cars
Least Efficient Two Seaters
Lamborghini Murcielago, 12 cyl, 6.5 L, Man(6)		8	13
Lamborghini Murcielago Roadster, 12 cyl, 6.5 L, Man(6)
Least Efficient Minicompact Cars
Aston Martin DB9 Coupe, 12 cyl, 5.9 L, Man(6)		10	16
Aston Martin DB9 Volante, 12 cyl, 5.9 L, Man(6)
Least Efficient Subcompact Cars
Bentley Continental GTC, 12 cyl, 6 L, Auto(6)		10	17
Least Efficient Compact Cars
Bentley Azure, 12 cyl, 6.7 L, Auto(S6)		9	15
Least Efficient Midsize Cars
Ferrari 612 Scaglietti, 12 cyl, 5.7 L, Auto(6)		9	16
Least Efficient Large Cars
Bentley Arnage RL, 8 cyl, 6.7 L, Auto(S6)		9	15
Least Efficient Small Station Wagons
Audi S4 Avant, 8 cyl, 4.2 L, Man(6)		13	20
Least Efficient Midsize Station Wagons
Mercedes-Benz E63 AMG, 8 cyl, 6.2 L, Auto(S7)		12	18