I voted for Jason's own demise in Part 4. For a long time, I would have said "Take your best shot" from Part 8. Even though the movie sucked, I bust a nut in the theater when that happened. My honorable mentions: Axe in head and Mrs. V.'s decap in Part 1, wheelchair and sex spear in Part 2, handstand in Part 3, shower head crush in Part 4, pruning shears in eyes in Part 5, Jason killing an RV in Part 6, the sleeping bag (also Kane Hodder's favorite) in Part 7, the tent kill in JGTH, and the frozen face-smash in JX (even if Mythbusters said it wouldn't work).
Seriously there's too many good ones to choose from. But to change things up I'm going all non weapon kills. I like the ones that show Jason's imposing strength. Friday the 13th Part VII: The New Blood - The sleeping bag kill. - When Jason crushes Ben's head. Friday the 13th Part VIII: Jason Takes Manhattan - When he strangled Eva and then threw her to the floor. - The gang member that gets his head bashed into the steam pipe. Jason X - The frozen head bash.