Perhaps my all-time favorite scene is the one from "Sport Fucking #2" where Manuel and Penny Flame are fucking. One of the reasons why I enjoy it so much is that, while Penny isn't necessarily one of my favorite performers, she seems really, genuinely aroused by Manuel's affects. It goes well beyond the usual BS moaning and groaning... there is tons of kissing, holding and two cum shots (after Manuel cums on her face, she asks him to keep fucking her).
Obviously, only the actors can attest as to the sincerity of a scene. For all I know, the scene above is just an exceptionally good performance (or great direction) and nothing more.
But that said, I'd like to hear some of your favorites... scenes that hold special value for you because of a perceived element that suggests genuine arousal on the part of the actress. Be sure to tell us why, as well (e.g. rather than simply say that its a hot scene, tell us what it is that the actress is doing that makes it seem that way).
Looking forward to the replies. Thanks.