The most massive blackhole to date has been found located in the M33 galaxy about 3 light years from Earth. The blackhole, part of the M33 X-7 binary system, is by far the largest blackhole ever found at approximately 15.7 times the mass of the Sun.
It also has a companion star; an unusually large star that is nearly 70 masses of the Sun. THe companion star in this binary system also hits the record books because it is the largest companion star to a blackhole ever recorded.
This system also eclipses regularly, and is the only known blackhole/star binary system to do so. This allows for unusually accurate measurements and the extreme nature of this binary system make it a great place to check out our current astrophysics models.
However, how such a large blackhole and companion have formed is very difficult to explain using current physics models....
Wonder if string theory has the answer to this one!