Wikipedia says that A Clockwork Orange was rated X, which I did not remember. I know that Last Tango in Paris was rated X.
One funny piece of ratings trivia is that a shorter version of Once Upon A Time In the West was original released in the US with an R-rating. When they put out the full version on DVD they submitted it for a rating and got a PG-13. So they got a lower rating for a longer version of the movie that included all the violence from the original R-rated version.