For me its The two mexicans with a chainsaw vid thats the worst I'v seen because I managed to watch it all the way through.
I've also been recommended to watch these:
"3 guys 1 hammer"
Luka Magnotta's "1 lunatic 1 icepick".
I only watched a little bit of the 3 guy and 1 hammer but quickly turned it off because I felt so disgusted n filthy for even watching it. Not gonna even bother with the Luka Magnotta's "1 lunatic 1 icepick" I heard that's the worse,