The achieve the highest possible luminosity, we need to start with the most energetically dense object, which is a black hole. The energy of a black hole is equivalent to its mass $ E = M c^2 $. The radius of such black hole is $ R = \frac{G M}{c^2} $. The smallest possible time in which all the energy can be released is $ t = \frac{R}{c} = \frac{G M}{c^3} $. Hence the highest possible luminosity is

$ L = \frac{E}{t} = \frac{c^5}{G} \approx 3.6 \cdot 10^{52} W $