The answer to this question actually depends on a lot of things. First, people use different types of fuel, and they have their preferences. This means you will get different answers depending on what type of fuel people think is best for them.
Another reason is that there are different reasons why we use fuel. That said, you can't compare fuel consumption in vehicles and other smaller machines. However, you can consider your fuel to be efficient if the fuel consumption is 6-litres per 100km. The fuel that allows you to travel the highest kilometer with 1 liter should be seen as the most efficient fuel.
If I should give a specific answer, my answer to this question would be Diesel. Vehicles that are powered with diesel seem to have higher energy efficiency compared to those that use petrol. Also, the type of engine that comes with your car might be something to consider when deciding the most efficient fuel for your vehicle.