via Minute Physics
This video is about how efficient various reactions are at converting mass to energy (as we know from the Einstein mass-energy equivalence of E=mc^2). Antimatter is very efficient but it is not naturally-occurring. Chemical reactions like fire or explosions are very inefficient. Nuclear fission and nuclear fusion are better, but not amazing on an absolute scale. Non-rotating black holes (Schwarzschild) and rotating (Kerr) are by far the most efficient, due to their accretion disks and very small radius of their innermost stable circular orbits.