All-electric cars are the most efficient ones among all types of drivetrains, followed by plug-in hybrids.
This fact is confirmed by the official Fueleconomy.gov data, which compares MPG for conventional cars with MPGe equivalent (33.7 kWh = 1 gal. of gasoline) for EVs, under three categories - combined & city/highway ratings.
Among the 10 best 2015 models, there are only all-electric cars, and the worst combined results were above 100 MPGe.
Top Fuel Sippers (EPA Ratings, 2015 Model Year)
When we switch to the another comparison, for all-years and exclude all-electric cars, we can see that BMW i3 is a double winner.
According to the EPA, Chevrolet Volt has just slightly better fuel economy than the Toyota Prius Plug-In Hybrid.
The best conventional hybrid cars in 2015 stand at 50 MPG, while Volts and other PHEVs often get a lot fuel economy in real world driving if all-electric mode is used often.
Top Fuel Sippers (EPA Ratings, All Years) (all-electric vehicles are excluded)