The most luminous emitters of electromagnetism in the cosmos are often said to be gamma-ray bursts, which are bursts of energy that last from several milliseconds to several minutes and are released when a supermassive star goes supernova. They’re so bright that they can release more energy in ten seconds than the Sun will in its 10 billion-year lifetime.
Aside from gamma-ray bursts, the other brightest objects in the universe are quasars. These are the areas directly around a black hole where stars, gas and light are being pulled inwards. It is estimated that some quasars emit more light than 100 galaxies.
Image courtesy of ESO / M. Kornmesser.