Some of the cars with the best fuel efficiency include:
Toyota Agya 1.0
Fiat 500
Renault Kwid 1.0
Suzuki has done well in this space, with the S-presso, Swift, Ignis
Crossovers with small diesel engines like the Kia Seltos 1.5 diesel are also VERY efficient
Watch out for any small engines with a turbo, as they will have great claimed figures for fuel efficiency but never come close in reality.