My #1 would probably be the argument between Ann-Margret and Jack Nicholson in Carnal Knowledge. It's viciously and insanely well-written and performed.
Jonathan: Talk about the pot and the kettle. When I caught wind of your checkered past, I felt like a celibate.
Bobbie: You made me tell you.
Jonathan: Sure. I twisted your arm.
Bobbie: It got you hot!
Jonathan: Something has to.