The largest SMBHs (Super-massive Black Holes) in the universe are truly gigantic. How they even got so big is a mystery, but we mostly think it was either by a long history of merging with other black holes, feeding on the intergalactic medium of gas and dust early in the universe's history, or by being primordial black holes that formed at the dawn of time itself.
The largest known is NGC 1277 and its event horizon would easily swallow the whole solar system.
@starwall Oooo, this thing amazes me. More specifically, NGC 1277 is the name of the galaxy, and this black hole sits at the centre of it. It weighs 17 billion solar masses, and makes up 14% of the total stellar mass of the galaxy.