Reading about the Andromeda–Milky Way collision, a galactic collision predicted to occur in about 4.5 billion years between the two galaxies, I went to the galaxy merger page and Wikipedia states that :
One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702.
The universe is a big place and wikipedia is not the most thorough with this subject.
What is the biggest merger we have found up till now?(29-09-2019)
Galaxy cluster MACS J0717, one of the most complex and distorted galaxy clusters known, is the site of a collision between four clusters. It is located about 5.4 billion light years away from Earth. Image credit: X-ray: NASA/CXC/SAO/van Weeren et al.; Optical: NASA/STScI; Radio: NSF/NRAO/VLA