Personally, in terms of non-irl stuff, its gotta be Teenage Mutant Skullfuckers for me. Somehow, that's a lot worse than all the scat videos and most gore i've seen. I think its mostly due to the extremely cheap budget and the deadpan eyes the turtles have...grosses me out!!
IRL-footage wise, I'd say the worst I've seen are WomensAlliance (I had to click out of that one!), FunkyTown gore (Literally lasted 10 seconds and i clicked right out), or 1 kid 3 hamsters. (awful "fun" fact, me and the guy in the video used to be on speaking terms due to both of us being horror artists a couple of years ago, I'm especially disgusted to see what he's turned into and knowing i spoke with him fairly often. The more ya know.)