Although I don't focus that much on the gore scenes and haven't seen part III, until now it would be this one:
https://www.youtube.com/watch?v=e2IcNXxl5zs
The whole sequence is hilarious though. The group get closer to Crowley's shag, real scary atmosphere, everybody's awaiting that Crowley appears out of nowhere or that someone just gets pulled into the woods – but then, BOOM, we just see Crowley in all his glory, he screams, hacks the guy in two halves and rips his wife's head apart.
I laughed my ass off during that scene.