It's hard to pick the most erotic movie, but I'll contribute one of my favorite scenes in a mainstream movie. In the movie
"History of Violence" there's a great example of how you can sell sex without really showing much at all. If you like fast, rough sex on a hard wooden staircase in dim lighting...check it out.
Note to Ramster: this one has a lot of action too.
Note to UW: tried to watch Brüno but couldn't do it for more than about 10 minutes. I guess I'm just not evolved enough.