Yeah, the absolute limit is c^2 if you've got 50-50 matter-antimatter. The efficiency however is brought down by the difficulty of _producing_ antimatter in the first place. A more popular hard-sci-fi concept is black hole engines; a 600kt black hole sprays out a hundred petawatts of Hawking radiation, and the fuel is whatever you've got lying around. That still has issues around energy _capture_, since the radiation is going to be ridiculously high-frequency. The best way I've heard of to dodge those issues is the 'Penrose process', where the fuel is the black hole's rotation and you get good ol' kinetic energy out. That can at most get you 20% of the mass-energy though, and there's a bunch more small print.