If you want to be #1 in the USA, you need an extreme vehicle. A four wheeled version of this:
HyperRocket: 125 mpg, 100+ mph 3-wheel motorcycle | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com would do it.
If you wanted something that's a normal car under the hood, the most efficient cars happen to be hybrids. You could do a hybrid-delete to a 2010 Prius or a first-gen Insight with CVT (maybe drop a CRX-HF motor in it?) and you'd end up with a pretty good car.
But if you add the criterion "wasn't originally a hybrid", then it's Basjoos' Aerocivic. Or make your own out of a CRX, which has a smaller frontal area and shorter overall length, and so lends itself better to being streamlined.
If aeromods are also out of the question, just buy one of the very good but unremarkable economy cars listed above.