Depends how your defining dirty...
You mean the thing that was probably the most sexually perverted? or the thing that grossed you out the most?
Nothing really grosses me out until you get to the things that you mentioned like scat and bestiality.
Scat is just vomit-inducing... and bestiality is off, though 'sometimes' intriguing at the same time.
Probably the thing that grossed me out the most was a guy getting his ass double-fisted by a guy while sucking off a chick with a strapon. And he was nearly elbows deep, that was bad, and I aint gay so it was freaky.
I saw this other one where an asian shemale is getting double-anal'ed too. Freaky. But the double fist takes the cake.
Those things grossed me out and were probably the most twisted, but most sexually perverted porn while still being cool to see, was probably just a couple of double anal penetrations on chicks by some HUGE guys. And a triple penetration (more of an attempt actually) but the guys werent big. Bukkake is pretty perverted I find too. I want to see a doubleanal-doublevaginal, but have never seen nor heard of any movies containing those. Anyone know any?