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Friday the 13th pt 2, hands down. One of the main reasons for this choice is regarding these two series, the second film always makes me consider the third as well.
In the friday series, part three fleshes out the main character introduced in part two, granting the mask that turns Jason into the iconic legend he is today.
In the halloween series... well, number three is the black sheep. Decent enough film, but the totally out of place nature of the film hurts itself and the series in general.