gas

On the highway I'd put the VW Jetta Diesel as one of the top 5 cars. For city the Toyota Prius will be in the top 5. Not sure which car is the mileage king this year.

The one carrying the most people -- i.e. passenger miles per gallon. A Mercedes S Class with five passengers does better with a Prius with just the driver.

Twotone

Both of the above are right. Well you did ask about gas not fuel mileage. Anyway best needs a better description. You need to consider the utility of the car for your specific needs, which may be far different than mine. You should consider the cost of the car. If you are looking for the most green car, then you need to look at emissions as well and that will end up comparing diesel vs gasoline emissions and there you have apples and oranges.

As always if you give us more information you will be a better answer.

I hear those 125c Scooters get over 100 MPG

I hear those 125c Scooters get over 100 MPG -- yes, but not in the snow.

The OP said car though.

It is the Prius.

EDIT: Glad to see you stopped looking at "air cars". A hybrid would suit you much better.