The largest black hole in the Universe JUN 16 2014
The black hole at the center of the Milky Way galaxy is estimated to have a mass of 4 million Suns. The largest black hole astronomers have found so far has a mass of 18 billion solar masses, or more than 4000 times as massive as the Milky Way's.
Around 3.5 billion light-years away, this galaxy is estimated to contain the largest black hole presently known, at 18 billion solar masses. (Although, the error bars for this one and NGC 1277's overlap substantially.) But the most spectacular part of this galaxy -- and why we're able to learn so much about it's central region -- is because there's a 100 million Solar mass black hole (that's 25 times larger than the one at the Milky Way's core) that's orbiting the even larger one!
Also, the largest know galaxy in the Universe is IC 1101, with a mass of 100 trillion solar masses.