3. Guido Boccaccini and Paola Reubens in Twitch of the Death Nerve (1971)
The late Italian horror master Mario Bava's many influences are regularly seen in damn near every genre film to come after his own reign of excellent cinematic terror, which ended with 1980's batshit Inferno. And no type of horror picture bears as heavy an amount of Bava cues as the slasher film, which the director all but invented, or at least perfected and revolutionized, with classics like Blood and Black Lace and 1971's Twitch of the Death Nerve.
In the latter, about family members getting gruesomely slaughtered while hoping to receive a lofty inheritance, Bava stages a brilliant piece of elaborate homicide that's been copied since, namely in Friday the 13th Part II. As two young lovers knock boots in bed, the killer rams a huge spear into both of their chests from under the mattress, simultaneously murdering them and, more importantly, making ready-for-whoopee viewers talk their sex partners out of having intercourse anywhere near a Serta product.