Nuclear Thermal Rockets
They are actual technology, we just don't use them because if the rocket has a failure and explodes, radioactive matter is thrown in the atmosphere. But if built and used only in space they would be really viable.
Their efficiency would be higher, with higher exhaust speeds, and it would not need oxidizers. 2 fuels would be hydrogen deuteride (HD), for high efficiency, and methane (CH4) for higher thrust (methane NTRs would be the engines with higher thrust in the game. They would need large radiators to dissipate the heat from the fission reaction.
Due to it's life-threatening nature, they would be prohibited to fly in atmosphere, but, if we had a moon building and launching facility or buildable bases, they could be built and used in space.