Fueleconomy.gov is the official source for light vehicle fuel economy ratings. The Web site has data extending back to 1984 and has recently published a list showing the highest-rated vehicles over this 26-year period. The list breaks down into two broad categories: (1) very small, light weight vehicles from the mid-80's/early 90's with few amenities, and (2) technologically advanced hybrid vehicles produced in 2000 and later. Top honors goes to the 2000 Honda Insight with a combined rating of 53 miles per gallon while the 2010 Toyota Prius, a midsized vehicle, takes second place.
Fueleconomy.gov's Top Ten EPA-Rated Fuel Sippers (1984 to present)
Supporting information for the information listed above can be found on the Fueleconomy.gov Web site.