Without doing any research, I'd think it was the Smart for 2 car.
Sadly, the smart fortwo or whatever they call it is more a product of marketing... They actually get pretty shitty mileage for a car that's main focus is being efficient... with an epa estimated 36mpg combined... Not bad, but for what you lose it's kinda a joke... I mean it's a go-kart and it can't beet what aircooled beetles did back in the day... Plus it 'needs' premium to get that mileage... (I hate the smart car... but that is my bias, I can't see any reason for 'em.) As to the original question... fueleconomy.gov puts the only '09 making more than 40mpg combined as the Civic hybrid with 42mpg. I think of recent years the Honda insight with the 52mpg combined is the highest... (Honda is king, IMO, when it comes to hybrid tech!) There are also the older Jetta Diesels that got decent mileage 38mpg, which got that while still being fairly practical. Granted you can play with math with the plug-in hybrid models and claim ridiculous mpg claims by comparing gas to electric costs... They like to ignore battery charge efficiency and regular discharge rates in those so be careful when you read those statistics...