NGC 6872. Source
This galaxy, NGC 6872, is the largest galaxy found thus far. It is a behemoth of a formation, spanning 522,000 light-years, more than five times the Milky Way's impressive 100,000 light-years.
This galaxy was big to begin with, but some time in the far past it collided with another, smaller galaxy and absorbed it, increasing NGC 6872's size. Not only that, the tidal forces from said collision pulled on the galaxy, forming two long arms that add greatly to the galaxy's length.
This galaxy may not necessarily be the most massive, but from end to end, it is, by far, the longest.