Ultimately the best setup possible to move through space in a normal fashion would be to create or mine a miniature black hole of roughly a billion metric tons, and utilize the Hawking radiation to convert mass to energy. The output would be a steady 350 Mw or so and the radius of the horizon would be roughly proton sized. One would need a confinement system similar to, but far more advanced than, particle accelerator beams used in research today. On paper you could even harvest mass along the way. Given the laws of physics, it is unlikely one could design a higher power thrust to weight of the drive system/craft over the long hauls of traditional interstellar travel.