what cars have the best gas mileage?

Hybrids, typically. Toyota Prius, Honda Insight, Hyundai Iconiq, Kia Niro, Ford C-Max, etc. If you're ok with slightly more limitations for range and whatnot, you could always go fully electric also. I'd really suggest that Google is your friend in this scenario. There's so much endless reading you can do for this type of research that I honestly think it's impossible to answer this question unless you tell us exactly what you're looking for in a vehicle besides having good fuel economy.

Small diesel cars don't use any gas at all but get really good diesel mileage. (in the 40's on the highway)