Mine is a scene from Teen Wolf (1985) when a girl drops her towel in front of teen wolf:
https://www.youtube.com/watch?v=YZ11hDE35QQ
As you can see (and from what I read online I believe many people agree with me), although the amount of sex in this scene is not very high, many people find it very arousing and memorable. Same thing cannot be said about many scenes which are more sexual but less arousing and memorable.
I hope I'm not just feeling this way because I'm a little old fashioned!
Do you have something similar which has stick in your mind for years?