Since nobody mentioned this already…
there's a web collection of movie scenes at princes-horror-central.com (which for some reason doesn't host that one clip of Renee Griffit's beheading in Showdown In Little Tokyo)

My absolute favorite is a tie between a strangling scene from Woman's Prison Massacre and the beheading from Showdown in Little Tokyo. They both have very good build ups, the woman is naked and in a sexual scenario - getting whacked at the high point. They think they'll have sex, but end up choked or decapitated.

This type of scenario is actually kind of common now that I think about it… Other worthy mentions being Ryde (two women want to make up with a dude and he ends up drowning one and head stomping the other), Appetites (good looking prostitutes get whacked, but there's actual sex scenes with a deathly climax), Murder Set Pieces (an overall deranged movie, that I dig) and others I can't remember from the top of my head - but if anyone knows this scenario unfolding in any other movie - *please notify me*.

There was also this one movie - the name of which I completely forgot - where a cute blondi is tied down to a chair by a clown / masked man and then he unveils another woman in front of her, completely naked and tied upside down, cunt up. He then fetches a chainsaw and there's a very good graphical scene where she's cut in half. The horror on both of them is delicious.
The blondie on the chair manages to escape from the restraints but she's shot down in another scene towards the end - with a kind of drawn out scene that ends with a headshot (if I remember correctly and my mind doesn't embelish such details as is the case with my memories of the little tokyo beheading).