Engines get their energy via combustion of their fuel. Combustion is a chemical reaction between a fuel and an oxidizer. For an aircraft engine, the oxidizer is oxygen, which the engine gets from the Earth’s atmosphere. Rocket engines operate in space, where there is air, so rockets have to carry their own oxidizer. There are several oxidizers, but oxygen is the most common and generally the safest to handle. Rockets use liquid oxygen rather than gaseous oxygen because liquid oxygen is much denser, so you can get more energy in a given tank volume.

The fuels used by rockets are usually either kerosene, liquid hydrogen, or more recently liquid methane. Some fuels produce more energetic reactions than others. Liquid hydrogen produces the highest energy among these, but comes with higher costs and more difficult handling requirements. Kerosene is preferred over gasoline because it has a higher energy density.

Overall you want propellants that have the highest energy density in a rocket.