Black holes are perhaps the most efficient bulk matter to energy conversion mechanisms that exist. Solar Fusion converts about 0.7% of matter into energy. A supernova will convert about 10% of it's mass into energy, but this is mostly in the form of neutrinos which are not very useful and is kind of a one shot deal.
Antimatter/matter annihilation is pretty good, but you have to start out with half your mass as antimatter and still lose about half to neutrinos.
A black hole, depending on how you use it, can convert 100% of any form of matter into energy. One method is via hawking radiation, a black hole will eventually radiate all its mass-energy as thermal black body radiation, you would have to have a small black hole though and carefully control its size and matter intake. A lot of tricky engineering.
However, the best method is quite simple and will work with any black hole, harvest energy from the accretion disc, as matter swirls into the black hole, it gets hot. very hot, hot enough to release the binding energy of atomic nuclei, effectively fissioning everything into a stream of quarks, kaons and whatnot. This process can radiate as much as 40% of the rest mass energy equivalent of whatever you throw in as energy.
to quote the wiki
This process of accretion is one of the most efficient energy-producing processes known; up to 40% of the rest mass of the accreted material can be emitted in radiation.
More details on how the energy is released and derivation of the above amounts is in this paper. Not for the physics adverse.