Which supermassive black hole is one of the most massive directly measured?

Among directly measured supermassive black holes, the one in the center of Holm 15A is often cited as one of the most massive known. Estimates place it at tens of billions of solar masses, making it far heavier than Sagittarius A*, the Milky Way’s central black hole at about 4 million solar masses. These objects are usually measured by tracking the motions of nearby stars or gas. Their enormous masses make them key to understanding how galaxies and black holes co-evolve.