Picking a favorite sex scene is a challenge indeed. As in real life, sex should not necessarily be confused with love, but the most beloved literary scenes for me combine the two.
Take the very first love scene in Diana Gabaldon's Outlander. Time-traveler Claire has been hastily married off to Highlander Jamie by his clan for political reasons, and a consummation is required to seal the deal.
Despite their growing attraction to each other, the situation is awkward. For one thing, Claire is already married in her other life—which (in a nice twist) makes her the experienced one, since Jamie is still a virgin. ("A virgin, not a monk," he reminds her.)
It's not the most rapturous coupling ever recorded; for one thing, it's all over in a hurry since he's so inexperienced. But the humor, passion, mutual affection and respect, as well as the intrepid sprit of collaboration that will serve them so well in all their adventures (and love/sex scenes) to come are beautifully established.
For those with a taste for something a bit less orthodox, try the elegantly written encounters between Kit Marley (Christopher Marlowe—recently deceased, but resurrected in the land of Faerie) and the Devil—yes, the Devil— in Hell and Earth, the second book The Stratford Man duology by historical fantasy author Elizabeth Bear.
Fraught with passion, powerful manipulation, complex drama, and dry wit, these scenes—ironically—serve the larger purpose of restoring to lapsed atheist Kit his belief in God.
The best sex and/or love scenes do serve the plot in these ways. Otherwise, a scene is "just sex," which tends to ghettoize a book into some lesser category of genre fiction.
And as Jennifer Barksdale Inclán points out, "bad sex" (bad for the participants, not badly written) can further the plot just as niftily as the other kind. When I sat on a panel about writing love scenes for a Historical Novel Society conference a few years ago, each of us panelists was asked to read a love/sex scene aloud. Because I thought my book's romantic love scenes were too deeply embedded in the context of the larger story, I opted to read about my heroine's "first time." It was a disapppointment for her, but an eye-opening glimpse into the character of her partner—for both her and the audience—that limned each of their personalities in a nutshell (or so I hoped), while launching my heroine into the next phase of her journey.
Some of my novelist friends dread writing sex scenes as a necessary chore. Not me; I love to write them. What better way to further your plot in some fiendishly clever way than when you're sure you've got your reader's full attention?