What is the most fuel efficient thruster?

Could you please elaborate on what factors contribute to a thruster's fuel efficiency, and what makes one more efficient than others? Is it the design of the thruster, the type of fuel it uses, or perhaps the conditions in which it operates? Additionally, could you provide some examples of highly efficient thrusters that are currently in use, and explain what makes them stand out in terms of fuel efficiency? Lastly, are there any emerging technologies or designs that promise even greater fuel efficiency in the future?