Part 6 & Part 8. Part 3 honorable mention.
I like the zombie that Jason became. Throwing knives, hunting knife and Machete. A zombie that uses throwing knives? I'm sold. Part 8, he looks gross and wet the whole movie, and I like the dumb jack-o-lantern style face at the end, I don't know why. Part 3 is the scariest appearance by far.