It depends on your state:
In 26 states, a plug-in hybrid is the most climate-friendly option (narrowly outperforming all-electrics in 11 states, assuming 50:50 split between between driving on gas and electric for the plug-in hybrids), and in the other 24 states, a gas-powered car is the best.
Drum explains the variation:
If you live in, say, Washington or Vermont, where most of your electricity comes from hydropower or nuclear, an electric car is pretty carbon friendly. If you live in Kentucky, where your power mostly comes from coal, an electric car isn’t such a good choice. But there’s more. You also need to account for the carbon emissions it takes to build the car in the first place. And since battery manufacturing is pretty carbon intensive, a car with a big battery starts out with a big carbon deficit to make up.