difeerent scenes come to mind...
some of these have been mentioned by other terb'sters earlier. For a woman by herself Kim Bassinger in 9 1/2 weeks. I think more dancers would increase business by dancing to Eurythmics "this city never sleeps" for the 3rd song LOL. For a lesbian scene Helen Shaver and Patricia Charbonneau in Desert Hearts is excellent.
Now here is a genre that hasn't been mentioned yet, scenes with actress playing a sex trade worker, eg massage attendant, dancer, escort etc.For a guy and girl clothed it would be the scene where Christopher Reeve is being straddled by Kathy Baker in Street Smarts (Mimi Rogers also in it but for Mimi see the Rapture or Full Body Massage to appreciate her physique). For guy-girl with nudity the lapdance that Elizabeth Berkley gives Kyle MacLaghlan in Showgirls, now THAT was a lapdance!